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461 views118 pagesAfter Man
Dougal Dixon (1981)
Uploaded by
Alberto FerrazCopyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
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/ 118
A ZOOLOGY OF THEFUTURE — |
BY DOUGAL DIXON
INTRODUCTION
BY DESMOND MORRIS
ST. MARTIN'S PRESS
NEW YORK Published in the United States of America in 1981 by
‘St. Martin's Press
175 Fifth Avenue
New York
NY 10010
Library of Congress Number 81-50345
ISBN 0-312.01163.6
© Harrow House Editions Limited 1981
For information, write: St. Martin's Press
{All rights reserved. No part of this work covered by the copyright hereon
may be reproduced or used in any form by any means ~ graphic, electronic,
‘or mechanical, including photocopying, recording, taping or information
storage and retrieval systems — without written permission ofthe publisher.
Edited designed and produced by
Harrow House Editions Limited
7a Langley Street, Covent Garden, London, WC2H 9J
Edited by James Somerville
Designed by David Fordham
Phototypeset by Tradespools Ltd,, Frome, England
ustrations originated by Gilchrist Bros. Ltd, Leeds, England
Printed and bound in the United States of Am CONTENTS
INTRODUCTION BY DESMOND MORRIS 9
AUTHOR'S INTRODUCTION 10
EVOLUTION 11
Cell Genetics : Natural Selection : Animal Behaviour : Form and Development :
Food Chains
HISTORY OF LIFE 22
The Origins of Life : Early Living Forms : The Age of Reptiles :
The Age of Mammals : The Age of Man
LIFE AFTER MAN 33
The World after Man
TEMPERATE WOODLANDS AND GRASSLANDS 36
‘The Rabbucks : The Predators : Creatures of the Undergrowth :
The Tree Dwellers : Nocturnal Animals : The Wetlands
CONIFEROUS FORESTS 50
The Browsing Mammals : The Hunters and the Hunted : Tree Life
TUNDRA AND THE POLAR REGIONS 58
The Migrants : The Meaching and its Enemies : The Polar Ocean :
‘The Souther Ocean : The Mountains
DESERTS : THE ARID LANDS 70
‘The Sand Dwellers : Large Desert Animals : The North American Deserts
TROPICAL GRASSLANDS 78
The Grass-eaters : Giants of the Plains : The Meat-eaters
TROPICAL FORESTS 86
The Tree-top Canopy : Living in the Trees : The Forest Floor :
Living with Water : Australian Forests : The Australian Forest Undergrowth
ISLANDS AND ISLAND CONTINENTS 100
South American Forests : South American Grasslands : The Island of Lemuria :
The Islands of Batavia : The Islands of Pacaus
FUTURE 113
The Destiny of Life
APPENDIX 117
Glossary : The Tree of Life : Index : Acknowledgements FOR
GAVIN
eto and pli, ot ew eres
he Wal etlby Fa
cen Fleming i
{revlon "Sf ats Led end Gary Mash
8 INTRODUCTION BY
DESMOND MORRIS
‘As soon as I saw this book, I wished I had written it myself. It is a marvellous idea,
beautifully presented. Many years ago, as a young zoologist, I started inventing imaginary
creatures, drawing and painting them as an enjoyable contrast to the demands of my
scientific studies. Released from the restrictions of evolution as it really is, I was able to
follow my own, private evolutionary whims. I could make monsters and strange
organisms, plant-growths and fabulous beasts of any colour, shape and size I liked, letting
them change and develop according to my own rules, giving my imagination full rein. 1
called them my biomorphs and they became as real to me as the animals and plants of the
natural world,
Dougal Dixon’s mind has obviously been working in a similar way, although the
creatures he has brought to life are very different from mine. Instead of inventing a
parallel evolution, as though it were taking place on another world, he has given himself
the intriguing task of contemplating a future evolution on our own planet, closely based
con species that exist at present. By waving a time-wand and eliminating today’s dominant
species, including man, he has been able to watch, through his mind's eye, the lesser
animals gradually taking over as the major occupants of the earth's surface.
Setting his scenario in the distant future, about 50 million years from now, he has given
the members of his new animal kingdom time to undergo dramatic changes in structure
and behaviour. But in doing this he has never allowed himself to become too outlandish,
in his invention. He has created his fauna of the future so painstakingly that each kind of
animal teaches us an important lesson about the known processes of past evolution —
about adaptation and specialization, convergence and radiation. By introducing us to
fictitious examples of these factual processes, his book is not only great fun to read but
also has real scientific value. The animals on these pages may be imaginary, but they
illustrate vividly a whole range of important biological principles. It is this — the way in
which he has perfectly balanced his vivid dreamings with a strict scientific discipline —
that makes his book so successful and his animals so convincing and, incidentally, so
superior to the often ridiculous monsters invented by the cheaper brands of Science
Fiction
‘The only danger in reading this delightful volume is that some of you may reach the
point where you suddenly feel saddened by the thought that the animals meticulously
depicted in it do not exist now. It would be so fascinating to be able to set off on an
expedition and watch them all through a pair of binoculars, moving about on the surface
of today's earth. Personally, I feel this very strongly as I turn the pages and there is
probably no greater praise that I can offer the author than that...
} Ae Wen AUTHORS
INTRODUCTION
Evolution is @ process of improvement. Hence, looking at the
animals and plants of today and their interactions ~ the delicate
balance between the flora, the herbivores and the meat-aters; the
precise engineering of the load-bearing structures of the grate’
backbone; the delicate sculpting of the monkey's foot, enabling it 10
rasp objects 2s well as to climb trees; the subtle coloration of the
puff adders skin, hiding it completely among the dead leaves ofthe
forest floor ~ and trying to project al of that into the fusue is @ near
impossibility. For how can you improve upon perfection?
(One trend that is foreseeable, however, isthe ruinous effect that
‘man is having on the precise balance of nature. I have taken this not
‘unjustifably to an extreme, with man having extinguished the
species that are already on the decline and having wreaked terrible
destruction on their natural habitats before dying out himself and
allowing evolution to get back to work, repairing his damage and
filling in the gaps left behind. The raw materials for ths reparation
are the kinds of animals that do well despite, or because of, mar’s
presence and which will outlive him ~ those that man regerds as
pests and vermin, These are more likely to survive than are the
highly modified and interbred domestic animals that he develops
1 and encourages to suit his own needs. The result is a zoology of the
world set, arbitrarily, 50 million years in the future, which I have
used to expound some of the basic principles of evolution and
ecology. The result is speculation built on fact. What I offer is not 2
firm prediction ~ more an exploration of possibilities.
‘The future work is described as iby a time-traveller from today
who has voyaged the world ofthat time and has studied its fauna
Such a traveller will have some knowledge of today’s animal life and
so he can describe things with reference to the types of animals that
will be familiar to the reader. His report is written in the present
tense as if addressed to fellow time-travellrs who have voyaged 10
the same period and wish to explore the world for themselves.
Sit back, fellow time-travellers, and enjoy the spectacle and
drama of the evolution of life on your planet.
Dougal Dixon
Wareham 1981
The shetches on this page ar selected from the authors cum working drawings
and were tse by the artists to prepare the plates and illustrations in After Man.
10 ‘The biclgial cel, shaun here in the press of eplizatng tel, is the fundamental
Imulding block that makes up all ving things. The clls capacity for infinite variation,
tthen taking part in sexual reproduction, is atthe root of evolutionary development.
‘The form and position of living things on earth can be attributed
to two things — evolution and environment. The study of evolution explores how life originated,
how it diversified the way it did and how different creatures have
developed from others. The study of a creature's environment (ecology) shows how
the various life forms interact with one another and how they interact with the environment they inhabit.
In other words evolution can be thought of as showing a longitudinal section
through the life of our planet while ecology shows the same situation in cross-section.
Each is inextricably entwined with the other and the two cannot be studied totally independently
‘Although both aspects deal with survival it should not be forgotten
] that extinction is a very important factor. Without it there would be no room for evolution
to take place. There would be no new ecological situations for nature to fill by the evolution of new
animals and plants from older stocks.
‘That evolution has taken place is apparent both from the fossil record
and from the evidence contained within living plants and animals.
Examination of fossil remains reveals a general development from the simple to the more
‘complex and also the part played by the environment
in shaping an organism to prevailing conditions. In living creatures,
comparability in structure, embryonic development and chemistry are powerful indications of similar
‘evolutionary history or of common ancestry.
Evolution is therefore not a process that has happened only in the past in order
to establish the animals and plants of today's ecology, but is a constantly continuing
process that we can study both from its results and from the fossil evidence of the past. It has happened,
it is happening now and it will continue to happen as long as life remains on this planet.
u EVOLU
GELBGE
| Animals, and indeed plants, are composed of microscopic bricks
called cells. The cells found in different organs and tissues of the
same creature are of quite different sizes and shapes ~ bones are
‘made from angular cells, kidneys from spherical cells, nerves from
Jong, narrow cells — but all are made from similar components,
Round the outside of each cell is a skin, the cell membrane
enclosing the gelatinous cytoplasm which carries a number of small
structures called organelles. The most important ofthese isthe cell
nucleus, which lies at the centre of the cell and carries the
information from which the entire organism is built
Most animal cells contain the same basic components. At the centre lis the
rrcles (A) which contains the cells genetic material. The mitachondrion (J
resportie for energy preduction, andthe [sosome(C), thch secretes chemical
raduets, le nearer the surface in the ytoplasm (D). The ribozmes (E), where
the proteins are asembled, lie along a conoluted structure of membranes nun
athe endoplasmic reticulum (F
‘This information is stored as a code, made up from a sequence of |
components contained in a long molecule of a complex substance
known as deoxyribonucleic acid (DNA). The DNA molecule is a
litle bit like a ladder that has been twisted throughout its length,
‘The shafts of the ladder are made up of sugar-phosphate molecules
land each rung consists of a pair of molecules known as nucleic-acid
bases. There are only four of these bases and the sequence in which
they are found along the twisted ladder gives the coded instructions
from which the whole organism is formed. Although repeated in its
entirety in the nucleus of each cell of the organism, only certain
parts of the code are needed to build up particular organs.
12
TION
NETICS
‘The peculiar thing about the DNA molecule is its ability to
reproduce itself. The molecule splits along its length and unwinds
so that each half ofthe ladder consists ofa shaft and a series of half-
rungs. The missing ladder halves are built fiom the pool of sugar-
phosphate bases, which is supplied by the creature's food and is
present in each cell nucleus. As each of the four types of nucleic
acid base in the strand attracts only a specific kind of nucleic acid
base to itself, when two new complete strands of DNA are formed
they are absolutely identical to each other in the sequence of their
‘components. This is the most important process involved in cell
multiplication and underlies the growth of all organisms.
However, to grow, organisms also require proteins in the form of
cither structural elements such as collagen, in the case of the
packing tissue between organs, or as enzymes which aid specific
biological processes, Although the production of proteins is carried
‘on outside the cell nucleus it is controlled by the DNA and is
produced in a way analogous to DNA replication. The messenger
that transmits the DNAs instructions to the protein production
centre, the ribosome, is a molecule known as RNA. It is formed
along partly “unzipped sections of DNA and differs only subtly
from it. The messenger RNA travels to the ribosome, where it links
‘up with another form of RNA, transfer RNA, which bears amino
7
1. Spum
2 Own
The sperm penetrates the exam (A) and comes toe alongside the ovum muceus
(B). The chromesomes of both sperm and ovum divide into separate strands
Imnaun as chromatids. Corresponding chromatids mow to oppote ends of the
‘ouuen(C), wher they ae surrounded by nuclear membranes (D). The structure
then splits into separate cel (E). Daring cll division, when new el are being formed, the DNA (A) contained
uithin the dividing cell uncips and forms new malecles of DNA along its free
‘es (B) from the nucleic acid base’ ard sugar phosphates contained inthe cell
trucles. To produce messenger RNA, the DNA comes apart partially (C) and
links with broadly similar material; the sugar phosphate backbone is slightly
different chemically and one ofthe muck aes ie substituted. The mesenger
[RNA moves tothe ribosames, where it links up tech transfer RNA, achich
caries aminoacids (D). The messenger RNA contain the code that ensues that
the transfer RIA i linked together nthe coret sequence to produce the cai
of amino acide that form the desived protein,
acids, It's from these amino acids that the proteins are formed. The
RNA molecules are merely code carriers and ensure thatthe amino
acid link together in the correct sequence to form the protein type
required, In this way DNA controls the workings ofthe whole cll
and hence of the whole organism.
“The DNA molecules in the cell nucleus are aggregated into
structures called chromosomes, and specifi groupings of nuclic-
acid base sequences on the DNA. give rise to specific traits in the
‘onganism, These groupings are called genes. Half the chromosomes
in a creature's cells, and hence half its genes, come from its mother
and half from its father. This is reflected in the alignment of the
chromosomes during cell division. ‘The chromosomes then are
arranged in pairs, motherdonated ones aligned with identical
father donated ones so that comparable genes ae side by side. Even
though each gene in a pair contributes to the determination of a
particular characteristic, one gene ofien masks the effect of another.
|As part of the reproduetion process special cells known as
gametes —that is eperma or eggs ~ containing ony haf the number
of chromosomes found in ordinary cll, are formed in the sex
organs. Although one chromosome fiom each pair is present in
cach gamete, none is identical to any of the chromosomes received
from either the mother or the father, but contains a mixture of
‘material from both parents. This characteristic of gamete chromo-
somes is primarily responsible for the variation between incvuals
of the same species that is seen in nature. During fertilization, the
{gametes unite with others from a second individual to produce a
‘complete cell, with the full number of chromosomes, which in turn
divides and builds up a completely new organism with genetic
characteristics derived from both parents.
“This, briefly, isthe sophisticated mechanism that enables plants
and animals to reproduce and pass on their distinctive traits from
tone generation to the next. It is small changes, or mutations, in the
genes involved inthis process that allow evolution to take place. A
Tutation results in a variation in the characteristics of the adult
conganism growing fom the cell containing the gene. In most cases
the change that takes place is harmful and gives the organism @
disadvantage in the competitive world outside. The organism
perishes and the mutant gene perishes with it. Occasionally
however the mutant gene produces a trait that gives the organism
distinct advantage in its fight for survival
“The variation in genetic make-up tha sexual reproduction makes
possible produces the range of characteristics that are found
throughout individuals of a single species. Natural selection, which
‘ay be thought of as the directional impetus of evolution, acts on
this variability, favouring certain characteristics and rejecting others
according to their survival merit
&
uf
vy
ie
Nuctele-ocid
A sugerphosphate
‘Messenger RNA
=}
B L
EVOLUTION
NATURAL SELECTION
Natural selection, resulting from the environmental conditions in The directional influence of natural selection is more evident
which an organism lives, can have one of three different influences when the environment itself changes. Under these circumstances
on a population. It can be stabilizing, directional or diversifying. evolutionary changes occur such as to give the impression that the
‘The stabilizing influence can be seen where conditions have organism is evolving along a set path with a particular goal in view
remained unchanged over long period of time. The resultant This is quite erroneous and arses from the fac that in the context
environment consequently supports a well-balanced population of _ ofits environment the most recent member of an evolutionary series
animals and plants in which evolutionary development is disadvan- always appears much better adapted than the earlier intermediate
tageous. Under such circumstances any change occurring ina plant stages which, where they are known, look half-formed and
or animal will bring it out of the environment’s neat, efficient, time- incomplete by comparison, even though they were equally well
honoured survival pattern and put the creature at @ disadvantage, adapted to the environment’ own earlier intermediate stages. An
eventually resulting in its extinction. Its more conservative contern- example of this isthe evolution of the horse, which developed from.
poraries on the other hand will survive. Animals that have been a small forest-living browser into larg, long-legged running grazer
subjected to stabilizing selection fora long period of time may seem as its environment altered from forest to open grassy plain. The
quite unspecialized and primitive compared with those of similar small changes that enabled it to deal most effectively with its
ancestry that have experienced a more eventful evolutionary history. changing environment were continually selected for throughout its
Often they are characterized by passive survival mechanisms such history and in this way the horse evolved
as heavy armour, or high fecundity to offset losses through The diversifying influence of natural selection takes effect when a
predation. ‘new environment is established offering a fresh range of food
Merychippus
25 milion years
Hyracotherium 60 milion years Mesohippus
40 million years
‘Ages dated from before
the time of man
Pliohippus|
10 million years
The ors’ earliest knaon ancestor, Hyracothesium, a small long-toed creature outer toe: disappeared, leaving a single horny hoo. Is legs became longer as it
mo bigger than a dog inhabited the extensive forest areas found on the exrth ented into a ful fledged running animal and its dentition and digestive system.
betucen 50 and 60 milion years before the Age of Man. As conditions became changed frm that ofa breuser to that ofa grazer as it diet ateed frm eaves
diner atthe end of the Tertiary and the wodland receded, the creature became to grass. The most important stractual changes occured about the time of
progressively beter adapted to Wie on the plains. ts fet changed radically; the Merychippus which appeared about 25 millon years ago.
4 {As the time of man, a chan of subspecies, or cline, existed around the North
oie sth the British leer Hack-hached gull Larus fuscus gral, and the
Brosh herring gull, Larus argentatus argentats, a end members. All
smeightourng species of the cline could interred with one another excepting the
fend members, which, by the te the chain was complete, ere too distantly
selsted 19 mate uith one another sucess
2) Bosh leser blck-backed gull, Lane acu gral, (2) Scandinavian leer
ck backed gl, Lau fs fet (3) Siberian toga gull Lar areas vege,
(4) Amercan herring gull Lana arenttu oithmanas, (5) Beith herring gull
Lens arenas arent
CACTUS | GROUND
‘From the orginal finch that arrived at the Galapags Islands from South
America, arcund jiften separate species evolved to fill the islands vacant
‘aloe niches ~ each specs with specialised characteristics suited to itso
individual diet. The finches fall beady into thre distinct groupe according to
habitat —cacts, tee and ground duller and difer mainly nthe shape ofthe
bail I is thought that to begin with Binds were scarce onthe island allowing the
finches to evolve forms suitable for all the eruironmental sts availabe,
(A) Platypia cassis (B) Cacti haba, () Camarhytchu paras, (D)
(Cammarbynhuspouper,(E)Praeonas moma, (F) Glide lacs, () Gepisa
{rts (8) Gein magrratis () Geass fincas, ) Gopsa comity (8)
(Cepia andre
| resources and living spaces. An animal species entering this
environment may well evolve different forms that ae specifically
adapted to each of thes living spaces, or ecological niches. In the
absence of competing animals these diferent forms will eventually
‘develop into completely new species. This isthe kind of thing that
happens when an island, or a group of islands, is thrown up by
volcanic activity in the open ocean. The unpopulated island is
slowly colonized by animals which gradually diversify into diferent
species to exploit the whole area effectively. The classic example of
evolutionary diversification is seen in the Galapagos Islands of the
Pacific Ocean, Early in their history a small finch arrived that
subsequently evolved into tree-lving, insect-ating forms, seed-
15
1 British lesser
black-becked gull
‘ating forms with heavy bills and a form that ate burrowing grubs
winkled out with cactus spines. The large number of resulting
species reflected the large number of ecological niches available on
the islands.
Birds, with their power of fight, are usually the first vertebrates
to reach a new island and consequently far-flung islands can usvally
be counted upon to produce an interesting bird fauna. Typical are
the heavy flightless birds, such as the moa, Dinomis, of New
Zealand, the dodo, Raphus, of Mauritius and the elephant bird,
Aepyomis, of Malagasy, all of which evolved in the absence of
sground-living predators. The intervening sea was an effective
barrier preventing interbreeding between the far-travelled individ-
vals that reached the island and the original stock back home. Such
barriers to interbreeding are necessary in the evolution of new
species.
Races or sub-species often co-exist in the same area, exploiting
slightly different environments or food resources but retaining the
ability to interbreed. They may even exist asa chain of sub-species
reaching from one region to another, each sub-species able to
interbreed with the next one to it. When the species atthe ends of
the chain are quite differen the chain is called a cline. Occasionally
a cline may form a ring, for example round a mountain range,
where the two end members, although next to one another and
related, are so different that no interbreeding is possible and are,
technically speaking, different species. This poses problems in
taxonomy since, as interbreeding is possible elsewhere throughout
the ring, the members must strictly be considered as sub-species of
the same species.
‘Once a group becomes isolated from its original population it
‘may develop on its own to such an extent that, ifthe isolating
barrier later disappears and the two populations once more
{ntermingle, inerbreeding is no longer possible. They are now, by
efinition, two different species. The differences are accentuated if
the new location the isolated group finds itself in is basically
‘unsuitable. The group will very quickly disappear except for maybe
a few individuals at the extremes of the species range that show
some slight affinity for the environment. ‘The species that then
develops will be descended from those few individuals that were
‘genetically different from the main population in the first place and
contained by chance genetic traits which made them innately more
likely to survive
Because organisms are capable of infinite variability and have an
inherent tendency to change when set in an unstable environment,
new species appear more rapidly when the environment is changing
quickly. Evolution is so efficent that no ecological niche is left
vacant for long. Something will always develop to fill i EVOLUTION
ANIMAL BEHAVIOUR
Evolution does not involve the conscious will of the organism. Nor
ddoes it happen through any adaptation that is forced on it by its
surroundings, or any strategy learned by the organism during its
letime being passed on by it to its offspring. It happens, simply,
because certain characteristics in an organism's genetic make-up are
ther selected for or selected against by the particular characteris
tics of environment in which it finds itself. The environment, in this
context, is the physical surroundings of the organism, such as the
topography, the temperature or the rainfall, and the other organ
jsms that coexist with it, both those that it feeds on and those that
feed on it
The rate of evolution has little to do with the rate at which
{genetic mutation occurs - the important factor isthe environment's
rate of change; the speed at which new pathways open up into
which new forms may evolve and develop.
‘As well as being responsible for structural and morphological
traits in an animal the genetic make-up of a cell also gives rise to
behavioural traits that allow an animal to interact with its
neighbours and with its environment in a way that ensures its
sunival.
Tt can be argued that the function of an organism is merely to
pass on its genes to the next generation. Evidence to support this
view can be drawn from patterns of behaviour seen in animals,
Behaviour is, simply, an animal’ active response to its environment,
and along with growth and repreduction is one of the factors that
defines a living thing
Birds croved together when a hawk appears, thus making it more
difficult for the hawk to seize an individual. Running herbivores
dodge about to escape a swifter predator so that it becomes
‘exhausted before catching any of them. Young birds stay close to
Pricanrt gh
My
c
The visual courship display of birds is an important part of a behaviowal
pattern that alo includes song; male bird song is designed to atact females as
tell as to deter rival male. Viual display may tle place independent with
the intention of attracting a mate. An indtidual, usually « male, pstues and
signal until it has sscumed the attentions of «potential mate. The pair then
splay 1 concert, each responding to the others gestures ith the object of
dscovering the other’ ilingness or readiness to mate. Many species rely on
resplendent plumage for diplay, In most cases the males are ostertaticusty
Jeathere, ichereas the females are drab by comparison. The movements and
‘gestures in ontship duplay are vaualy those aseiatad with aggresion or
‘Gppeasement In some species preening and mock sleep areal pat of display.
(8) Gannet, Mout tossns, (B) Suge grouse, Centmcrs wophasans, (C)
(Corcrnt Phalanx cr, (D) Brolga ean, Gras matcun, (2) Great eee
rb, Paps cst (F) Addie penguins, Pres ali
16 cre
SANA MA a
their mothers until they are mature enough to fend for themselves
These, lke all aspects of behaviour, have evolved to aid survival. A
gene that introduces a behaviour pattern that does not contribute to
the survival of the species is soon eliminated
Courtship rituals are a very complex aspect of behaviour. The
act motion of a bird in a display dance or the movement of a
lizards head as it approaches a prospective mate indicates to its
future partner that it is in breeding condition and that it is a
member of the correct species. The latter point is important, for
although mating between two related but separate species may
produce offspring, they will almost certainly be sterile. Such
matings area tral waste of time and effort from the point of view of
‘eolution, as they do not successfully propagate the creature's genes
and are therefore to be avoided.
‘These activities are all instinctive hereditary behaviour patterns.
‘Other behaviour pattems are learmed and are also ultimately derived.
from the animals genetic make-up. The ability to establish the
appropriate action by trial and error, or by the example of others
around it, is an ability conferred on an animal by its genes.
Aggression is an element of behaviour that is perhaps more
complicated than it fist appears. One might ask why, if the object
of aggression isto remove one's competitors, do not animals fight to
the death each time there isa conflict? Apart from the obvious risk
involved, the answer is probably that, as an animal has no chance
‘of kiling all its potential rivals, by killing an isolated one i is just as
likely to assist its competitors as to benefit itself. In most cases
‘combat in the animal world takes the form of mock battles and
agaressve displays which do litte physical damage to the creatures,
involved, but do establish the dominance of one or other of the
participants. Thus the animal that wins a contest achieves what it
has set out to do, that is to gain or retain the resource in dispute
‘without suffering injury itself The loser also derives benefit in that
he escapes serious injury and retains the possibilty of contesting
future issues, where he may eventually be successful. It is difficult to
see how this strategy could be learned and itis more likely that i is
the product of evolutionary development; those animals adopting
the strategy are more likely to reproduce and therefore the genes
responsible for the behaviour are passed on in preference to others
that result in less successful behavioural patterns.
‘Throughout the animal kingdom behaviour patterns are designed
to ensure the survival of the individual's genes rather than the
survival ofthe individual. Loyalty is shown to the closest relatives,
since the loser the relative the larger the numberof similar genes in
its make-up.
‘The protective instinct which causes a mother bird to put itself in
danger or even sacrifice its life in order to save its brood is a
ation
Chatfineh
Fringlla coolebs
a
Imesigatons into the sng of the hafich, Fig cores, have provided
fascinating insight into the role of laing in beri. I at found ot
indicted on the sound specograph shun oposite, thet yang chair
reared in soltion were capable of oly a adimentary song that to produce
the fully developed form they had frit © hear the ong of others inthe al.
Uce mordax:
‘Male Fidler eras, Uca spp, attract mates by waving their large file claws
The gestures they make both inthe shape ofthe movement and nits peed wares
Letwven species living in the same area and ensue that ony female of the
comet species ave attracted. As only matings Beto individuals ofthe same
species ae likely to produce fertile offpring, thse males rot pssesing the genes
that produce the correct waving paters are ley to disappear.
behavioural trait calculated to promote the survival ofits own genes.
AAs the genes of the mother bird are present in the brood and the
several members of the brood have a better chance of reproducing
and spreading their genes than has the single parent bird, iis tothe
advantage of her own genes to preserve the lives of her chicks even
at the expense of her own Less obvious is the gene-survval
behaviour of social insects, such as bees and ants. A member of
such a group will fight to the death indiscriminately to ensure the
survivl ofthe colony. In this case members ofthe colony are much
more closely related to one another in genetic make-up than are
‘ther animals within a single breeding population. The survival of
the colony therefore ensures the survival of the individual's genes
despite the death of the individual.
‘Many mating ploys, particulary those seen in birds, may seem
actualy to reduce the individuals chance of survival rather than to
increase them. The breeding plumage of many male birds, as well
as being attractive to a mate, makes them visible to predators. Birds
possessing particularly long and spectacular tail feathers must find
them a great disadvantage when escaping fom a predator. It is
possible such handicaps to survival may be devices to show just how
successful the male is ~ if it can survive with all that working against
it, then it must be good! Hence the female i instinctively atracted
to the male that puts on the most extravagant display. EVOLUTION
FORM AND DEVELOPMENT
‘Natural selection lays down rules about precisely which form of life
js most suitable for colonizing a particular environment. This
evolutionary feature can give rise to a large number of different
‘animals with the same superficial appearance. When the animals
‘concerned have evolved from the same ancestor and have developed
independently along similar evolutionary lines, they are said to have
evolved in parallel, When the ancestors are different and the
animals have evolved along quite different lines to produce the same
final shape, their evolution is called convergent. An example of
pparallel evolution can be seen in the development of Equus, the
horse, which appeared at the end of the Tertiary in North America,
‘and Thoatherium, a remarkably similar ungulate which evolved at
the same time in the then isolated continent of South America. The
‘wo forms developed independently along similar lines from similar
‘ungulate ancestors in response to the same set of environmental
(Of the shark, fl-izard and deiphin, only the shark has exo from a marine
creature. The fh-izad and dolphin were evolied fom a landing reptile and
smaronal respectively. Despite their radically diferent ancestry they have all
adeped the seme streamlined form wo suit their quatic made of if and together
form a inking example of convergent evcuton.
conditions. An example of convergent evolution is found in the
development of the shark, Carckarodon, the fish-lizard, Ichthy-
‘xaurss, and the dolphin, Delphinus ~ three animals from totally
diferent classes but having adepted the same streamlined shape,
‘swimming fins and tail in order to exploit the same niche in the
‘same environment, that of active fish-eaters in the sea
One consequence of particular animal shapes fitting particular
ecological niches is that widely separated places with the same
climatic and environmental conditions may support very similar
faunas even though they have evolved fom different stocks. The
tropical grasslands of South America, Africa and Australia all at one
time supported animals with similar physical characteristics ~long-
legged, running grazers, swift carnivores, burrowing insectivores
and slow-moving heavy browsers. In Australia they were marsupial,
in Africa placental and in South America of both types. Despite
their differing ancestry many of these creatures were outwardly
similar. Such situations arise not only in diferent places atthe same
time, but aso in different places at different times.
The influence of latitude on animal shape and form has two
‘oddly contrasting effects. One known as Bergman's rule predicts
that, within related groups, animals living nearer the poles will be
larger. The other, Allen's rule, states that, again in related groups,
those living nearer the poles will have smaller extremities. Both
effects are heat-conservation measures designed on the one hand to
preserve body temperature and on the other to prevent frostbite.
Genetic changes may be minor and quite imperceptible or they
may result in changes that alter the species dramatically. The land
snail, Cepaea memoralis, lives in a variety of habitats in the
‘temperate woodland and can have any of several different shell
markings. Where the ground is open and grassy a plain yellow
coloration disguises the snail best and snails with other markings are
‘easily seen by predators and quickly devoured. Where the ground is
‘covered by leaf litter, brown striped forms are better camouflaged
‘and other forms are selected ageinst. This gives rise to populations
Of predominantly yellow snails in open grassy areas and brown
striped snails in woodland. A similar effect was observed in the
peppered moth, Biston betulaia, during the early days of man's
industrial revolution. Up until then the species had consisted largely
of grey and white speckled individuals which were perfectly
camouflaged against the lichen-covered tree trunks where they
lived, A black form also found in the population was easily seen and.
eaten by birds and was therefore uncommon, With the arrival of
heavy industry the tres became caked with soot and turned black,
affording a perfect camouflage background for the black form. The
white form was then selected against by predators and the moth,
population became predominantly black. Later, with the coming of
clean air laws, the atmosphere and the tree trunks became less soot-
laden and the moth population swung back to give a bias towards
‘white and grey individuals once more. These changes involved only
varieties within the same breeding population and there was at all
times a constant exchange of genetic material as they took place. If,
however, the environmental changes had been permanent and the
18 In laking atthe life on the graslands of Africa and Australia around the tine
‘of man and comparing it with the fe that exited on the plains of Scuth
“America sme tine earlier, during the middle Tertiary, we can se that animals
ith similar Wf styles appear to evolve similar shapes and sizes in corespording
environments It mahes no difference whether these eruironments ave separated
by time, space or bth, they are by far the most important single eoluionary
{actor gnering the shape and form of living creature. Large herbivorous
‘animals, wry similar in appearance to the rhinoceros, en long-legsed, sf
running grazing animals appeared in all thee environments, Camivres,
Insectitores and emnivores al specially similar to one another evolved. The
‘mos stringy similar groups were the burrowing insect-eters and the fights
birds, which because of ther highly specialized medet of hfe develeped along
rowdy the same line.
‘Changes in the environment due to the industrial revolution gave the black
stant forms (B and C) of the peppered moth, Biston betula, an advantage
smurbon areas, where they largely replaced the previously predominant grey and
tehte speckled fom (A). Because ofthe lw vel of atmospheric pollution, the
pepulation in rural areas was for the most port unaffected.
different varieties had become isolated from one another, they
‘would have in time become different species.
“Mimicry is a separate imitative phenomenon in which a creature,
‘usually for reasons of defence, takes on the physical appearance of
another animalorof aplantorindeed of totally inanimate object ikea
Bird dropping, In the case of animals mimicking other animals there
aze two important forms. The first, known as Mulleian mimicry,
fccurs when a number of dangerous or unpalatable species evolve
the same coloration or patterning to gain protection by association.
Animals exhibiting this form tend to have vivid colours which make
them stand out against the background and act as a warning, The
second form, Batesian mimicry, involves totally harmless creatures
adopting the coloration or appearance of inedible or dangerous
species in order to take advantage of their warning coloration and
© escape predation. Other forms of mimicry exist that enable
predators to approach prey which they themselves mimic. The
tesects and in particular the butterflies with their striking wing
pattems are the masters of mimicry, bu itis also found among the
vertebrates and among the plan's
As we have seen the rate of evolution is largely dependent on the
rate of change of the environment rather than on any trait possessed.
by the animal itself Even so it seems that the higher a creature is
situated on the evolutionary ladder the more rapidly it evolves. For
ample, bivalve genera exist for, on average, about 80 milion
years, ish genera for 30 milion years and ungulate and carnivore
genera for six to eight millon years. The shorter the life-span of a
genus the more quickly another evolves to take its place. This
results in a larger turnover of genera in land-based habitats, where
life on the whole is more highly evolved than in the sea.
Plants tend to evolve much more slowly than animals and the
flora existing during the Age of Man consisted mainly of plants that
exolved at the beginning of the Cretaceous period while the
dinosaurs were still the dominant form of land animal
ee
LEONA ARLE ea
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z ‘Stegotherium Aardvark ‘Numbat
bp & &
19 EVOLUTION
FOOD CHAINS
‘The food chain is a fundamental concept in ecology and is the
sequence in which organisms eat one another. It is more appropri
ate to regard the process as a pyramid rather than a chain, since in
any environment there are many more animals lying at the lower
tends of the chains than at the top.
‘The base members of the pyramid are the plants, the primary
producers, which use the sun's energy to synthesize raw food from
‘carbon dioxide in the air and minerals in the sol. From the myriad,
‘members ofthis broad base all food chains weave upwards towards
the camivorous animals at the apex. For example, in the far north,
luring the Age of Man, the plants that grew in the brief summers
were fed upon by insects, which were eaten by small birds, which
‘were in tur eaten by small carnivores such as foxes, which were
ultimately eaten by large carnivores such as polar bears. Similarly
the microscopic plant plankton existing in the sea at that time lay at
the base of a food chain which extended upwards through fish and.
seals and again to the polar bear. Nothing hunted live polar bear,
although once dead, scavengers and micro-organisms from lower
down the food chain fed on the carcase, reducing it ultimately to
the inorgunie substances on which the plants at the base of the
pyramid feed. Except in the worldof parasites, where the number of
Organisms supported at each stage increases rather than decreases,
food pyramids like these can be constructed for every type of habitat
fon earth, with in each case a single predator or small group of
predators lying at the top.
“The general layers in the pyramids are the primary producers
already mentioned, the herbivores and the carnivores. Throughout
the pyramid both scavengers and microscopic decomposers operate.
Ifone of the key members ofa food pyramid layer were removed by
disease or environmental change the structure would become
At the top of every fd pyramid sit the carnivores, the lst ink in an energy
transfer chain that Begins uth the plants ~ the initial fod synthesizers. This
foc in te for of lanes a frit passed onto the herbivores, lying higher up
the pram, and utmalely through them to the carivores. Similar pyramids
‘exit throughout the word in all environments from the tropics to the poles
Sometimes « predator, repreentd here by the polar Bear, may le at the top ofa
pyramid thal embraces both land- and soater-based ergonioms. The complet
Feeding relationships which exit between plants and animals ving together in
the same environment amount to 4 sefeuffiient organization fren as an
‘ecosystem, Ecosystems in topical areas may const of thousards of species.
20 Ir feld of clover (A, itis interesting 0 speculate what might happen fone ier
ofthe fond pyramid were removed. Ifthe oles were largely wiped out by disease
(By, the cls would be deprived of prey and soon laa (C), causing the insect
population to expand uncontrollably (D). This ntuation is unlkly to lat and
‘be vacant niche would rapidly become woceuped in one of thre tay totally
fnew inect-oatng creature such asa bind would arrive, bringing uaith i ts un
_Predctor (or another specie of ole would invade, bringing back the xl (F),
fora remnant of the orginal vole population, resistant to the diease, woul
restate itself (Gh
unstable. The species lying below the vacant slot would increase
‘unchecked to a point where they outran their food supply and their
‘numbers would be controlled by starvation. In realty this seldom.
happens and another predator Soon appears capable of filling the
‘unoccupied niche,
Plants can only use so much of the energy they absorb from the
sun. Itis difficult to measure, but certainly no more than an eighth,
of one per cent of the sunlight falling on a plant ean be stored by
being converted into sugar. ‘The chemical energy in the sugar is
sed by the plant to build up the complicated organic compounds
that go to form its structure. It is this sugar, and the energy
contained in it, that a herbivorous animal obtains when it eats grass.
However, it cannot convert al the plant’ stored energy into its own.
requirements ~ the maximum efficiency of an animal is about ten
per cent. This ten per cent factor is present at all stages of the food.
chain and means thet in any environment a hundred herbivores can
support only ten carnivores, and these ten carnivores can in turn
support only one “second-stage” carnivore, These figures are
‘oversimplified and refer to animals ofthe same size. The important
factor is the weight of the animals rather than the numbers of
individuals. The ten per cent factor holds true for every stage inthe
complicated pattern of food chains and is an important factor
leading to the stable shape of the food pyramid.
‘The dependency of feeding efficiency on sunlight is the reason
‘why different parts ofthe earth support quite different numbers of
Corganisms. In the tropics, where the sunlight i intense, much more
solar energy is available to be absorbed by plants. Hence, where
‘other factors such as rainfall allow, there is more vegetation per unit
area than in temperate or polar climates. This lage amount of plant
‘material is able to support a large number of herbivorous animals,
‘hich in tum support a large number of carnivores. In the Arctic,
con the other hand, the low level of solar energy produces @ much
‘more sparse vegetation, and hence there are fewer herbivores and
even fewer predators.
“The variety of species at each level in the pyramid depends on
the variety of plants at the base. On tropical grassland, for example,
‘here there are short grasses, herbs, tall grasses, bushes and trees,
cach of the large number of indigenous animal species eats a
different collection of plants. Therefore the animal that eats roots
does not compete with the animal that eats the low hetbs or the
‘animal that eats the tall grasses. Even those that do have a broadly
similar dict are sufficiently different in some way so as not to
‘compete directly —for example, one may eatin the daytime and the
‘other at night. In this way the ecological niches are multiplied and
the processes of evolution ensure they are all filled.
‘The principle that ‘nature abhors a vacuum’ is as true in biology
as itis in physics. An ecological niche is never left vacant for long
and something will evolve to occupy it as soon as one appears
Within each species, however, competition is strong and each
particular niche will support only so many individuals, Struggles
between members of the same species are usually formalized into
stylized displays in which litle real damage is done. Territory is
preserved and mates are chosen without recourse to any actual
‘Asa rough rule of thumb a predator requiring one unit of energy for subsistence
needs to tal in ter equivalent units of energy from the herbivores on which it
rey. Silrly each herbivore needs to receive ten units rom the wepettion.
‘The vegetation’ energy is derived solely from the sun, ard again, of ten rts
asorbed by a plant, ro more thon one uit i used effectively.
combat. This appears to be the strategy that leads to greatest
success in maintaining a creature's position in the ecosystem.
‘The predations of carnivores do little to upset the balance of the
food pyramid. By preying only upon the weak, sick and elderly ~ a
ppractce forced upon them by the fact that a healthy adult can
‘usually outrun or fight off an attack —it ensures that only the fittest
survive. Ifthe fit, healthy adults ofa species cannot outrun or fight
off an assault their species will swiftly become extinct and the niche
will be taken by another creature. In this respect predators can be
thought of as no more than impatient scavengers.
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22 HISTOR
| see
‘The map shous the configuration ofthe continents atthe beginning of the Cambrian, the
point in time from which the history of Wife can be traced uith some certainty. In |
Precambrian times most animals were sjt-bodied and are only rarely preserved.
The earth has existed for about 5000 million years and has been
populated by organisms of one sort or another for between 3500-4000 million years of that time.
However, an accurate fossil record of the earth's
life can only be traced back some 620 million years — to the time when hard skeletons
first came into existence. At that time life was present only in the sea and the land was barren,
‘The distribution of land and sea was not as it is today. The configuration
of the continents and oceans is constantly changing due to a mechanism called plate
tectonics. The earth's crust is made up of a number of plates, like the panels of a football.
These plates are formed continually along one edge, where material wells up from the earth's interior,
and are destroyed at another, where one plate slides beneath a neighbouring one and is lost. |
The upwelling takes place along mid-oceanic ridges and the destruction occurs along deep-sea |
troughs. ‘The material involved consists of oceanic crust, rich in silica and magnesia.
‘The continents are made of a different sort of crust, rich in silica and aluminium, which, being |
lighter, remains on top so that the continents are carried here and there over the
globe by plate-tectonic activity. This process has continued throughout geological time and |
will continue until the end of the world. The importance of plate tectonics to the
history of life on earth is not just one of geography. |
Plate tectonics in part affects the pattern of global climate, which in geological terms fluctuates |
over a comparatively short space of time, and has undoubtedly |
contributed to the relatively sudden changes that have occurred in the predominant life form on earth.
‘The juxtaposition of continents at crucial stages in the development of these animals has at
certain times been important in their spread throughout the world
and has produced marked differences between forms found on different land masses.
23 HISTORY OF LIFE
THE ORIGINS OF LIFE
‘The sun and the solar system were formed from a vast shapeless
cloud of interstellar gas, spinning slowly in space at arate of about
‘once in ten million years. As it rotated it began to contract under
the influence of its own gravity and, in consequence, to rotate more
rapidly. The forces involved flattened the gas cloud into a disc in
which material became concentrated at the centre to form the sun.
Across the disc, eddies appeared that began to accrete material,
forming the foundations of what later became the planets. Dust
particles consisting mainly of droplets of iron and particles of silica
compounds began to solidify. The droplets coalesced into lumps
and collected together in the eddies under the influence of gravity.
“The iron, being heavier, sank to the middle and the silica remained
‘on the outside to give the protoplanets an iron core surrounded by a
stony mantle, The inner planets ~ Mercury, Venus, Earth and Mars
— were fejmed in this way. The other planets aggregated from
lighter material such as carbon dioxide and ammonia, which
‘condensed fiom the gas as the temperature continued to fall. At this,
time the compaction of materials in the early sun triggered off the
process of nuclear fusion and the sun began to radiate energy ~ a
[process that has continued for the last 5000 million years and will
continue for 5000 million years to come
[in the day before proper scientific ivetigation, man believed thatthe earth
he ni and all the living creature ith which he wae familiar were the result
of asngle supernatural act of eration that had been carried out ata particular
dete inthe relatively recent past. Fosil sea creatures fund for in land, which
were later o provide evidence for major change in the distribution of land and
sea, were dismissed as being the esl ofa punitive fod.
Te is possible that the earth’ first atmosphere was rich in
hydrogen, methane and ammonia, similar in composition to the
atmospheres of the outer planets. As time went on water vapour
‘and catbon dioxide would have been added to these gases by
‘outgassing from the newly formed rocks. The water at first would
have remained as a vapour since the heat ofthe atmosphere at this
time would not have allowed it to condense. On the other hand, itis
‘equally possible that the primeval atmosphere of hydrogen
‘methane and ammonia was mostly driven away by the heat of the
sun soon after formation and thatthe earth’s frst stable atmosphere
was composed chiefly of carbon dioxide and water vapour vented
fom the interior through fumaroles and volcanoes. In ether case
the water that condensed and fell as rain when the earth became
cool undoubtedly contained molecules of ammonia, methane and
hydrogen dissolved in it. If chs solution was subjected to high-
energy influxes such as lightning bolts or ultraviolet radiation from
the sun, chemical reactions would have occurred that would have
synthesized complex organic molecules such as amino acids ~ the
materials from which living things are built
‘On the other hand there may be a totally different explanation for
the origin of complex organic molecules. Simple organic com-
pounds such as formaldehyde are present in interstellar dust —
Particles of carbon produced in stellar explosions. Molecules of
these organic substances may have accumulated on the particles and.
have subsequently united into the long chemical chains of complex
organic molecules that represent the fist step in the chemistry of
lie, Gas emitted from stars may contain oxygen, carbon and
nitrogen. Ifthe gas contains more oxygen than carbon or nitrogen,
corganic molecules such as polysaccharides (simple sugars) may
form. If nitrogen is the most abundant element the production of
nucleic acids and chlorophyll ~ the energiaing substance of growing
plants is more likely. Interstellar dust ean, under the influence of
gravitational forces, clump together, and in certain circumstances
fall into orbit around a sun as a comet. such a comet struck the
‘arth in the early days of the planet's formation, as is more than
likely, interstellar organic molecules would have reached the surface
of our planet.
‘Whatever the cas, itis certain thatthe hot seas on the steaming
surface of the earth 4500 milion years ago contained the complex
‘organic molecules that are necessary forthe building and develop-
‘ment of living things.
“The first thing on earth that could properly be termed ‘alive was
aa molecule with the unique property of reproducing itself, To do
this it must have been able to break down complex molecules such
as polysaccharides and use their constituent pars to build a mirror
image of itself Any characteristic ofthe basic molecule that helped
By Given suitable comonpheric and narfac conditions ts sil fore text on
«plant that als nth rou the Sun war as the ecsphere. Te elt
‘vende fom jut inside the orbit of Venus to jst aus the ot of Mar
Pivio 59000 “Merauy sth a maximum aaface tepertie of 370°C much to hot
‘Average distance from the Sun
in millions of kilometros
“upport Ife, and the outer planets, becoming progesively coder though to
Neptune aed Pluto where the maximum temperature is tell les ~200°C, are
such too cod
‘tin this ask would have enhanced its chances of survival and that
characteristic would have been perpetuated in the replication
process. Any feature that hindered it would have led to that
‘molecule’ extinction. Evolution had begun.
This activity continued until all the original polysaccharides
present in the primeval ‘soup’ had been used up. ‘The proto-
brganisms would have then run out of food had they not evelved the
ability to synthesize their own from inorganic substances using the
sun's energy. This process, known as photosynthesis, was made
possible by the presence ofthe chlorophyll molecules.
Eventually more than one complex molecule became involved in
cach replicating body and there appeared the compact organic unit
known as the cel. Some of the most primitive cells lacked a central
ancleus, the site of the cells’ reproductive machinery, and this
function was instead spread throughout the cytoplasm. Tt was the
cells with nucké, however, that were to go on to greater things, and
in the course of evolution smaller cells became incorporated into
larger ones, remaining there to perform certain vital intercellular
functions. Eventually complex structures arose consisting of more
chan one cell, each cell having its own particular role to play in
keeping the whole unit alive. The organism had evolved.
“The evolution ofthe frst multi-clled organisms may have come
about in one of two possible ways. ither by free-living cells of
Jupwer 7783 different types coming together a a single unit, orby cells failing to
separate completely during subdivision and remaining together as a
complex entity. Regardless of their formation, these multiple-celled
organisms must as whole unis have been more successful than the
sum of thei parts or they would not have survived
“The cells of multicelle creatures are not identical and have
quite different functions depending on the tissues or organs they
Constitute, In the higher forms of ie, some are structural elements
such as bone eell, others suchas blood cells provide defence aguinst
disease and transport food, whereas others such as nerve cells form
the organism’ sensory and communications system. Cel diferenti-
ation in most cases occurs atthe embrionic stage. To begin with an
‘embryo's cells are all identical. The initial fertilized cell divides into |
two caughter cells which divide into four cells and so on until
several hundreds, of identical cells have been produced. However at
4 particular point in the embryo's development this stage ceases and
specific cll are produced that are designed to fulfil definite roles. t
is unclear how this cell difereniation occurs. ll ell nuclei contain
the same genetic information, but only part of itis used in the
production of a new cell. Some agent within the cell, most likely |}
within the nucleus itself, must determine which piece ofthe genetic |}
cade is used to produce the new cell so that ican fll the Function
allotted to it
Uranus 20696
25 HISTORY OF LIFE
EARLY LIVI
‘Throughout the early oceans single-celled and mult-celled organ
isms, both plants and animals flourished. The plants were able to
absorb energy from the sun and to photosynthesize food from
inorganic material. The animals unable to produce their own food.
directly from sunlight obtained energy by eating plants. This
contrast in feeding methods is the basic difference between plants
and animals, and is reflected in the structure and physiology of the
‘wo types of organism, Plants, needing only sunlight and inorganic
materials, have no need to move if situated in favourable positions,
‘and theie cells are therefore stiff walled and rigid. They have fat,
‘energy-absorbing surfaces (leaves) which orientate towards the sun,
and anchoring structures (roots) through which they absorb
nutrients and which also prevent them from being blown or washed
away. Animals on the other hand, need in most cases to move fom
‘one plant to another and have therefore evolved more flexible cell
walls and muscular systems to make movement possible. They have
developed sensory organs and nervous systems through which they
‘evaluate their surroundings and by which they transmit messages to
their muscles
‘Associated with its power of movement is an animal’ overall
‘geometry, Those that are not just shapeless sedentary lumps
filtering food from passing water currents have a symmetry that is
either radial or bilateral.
[At the beginning of the Cambrian period hard-shelled animals
appeared for the first time in large numbers. As normally only a
creature's shell becomes fossilized, the history of life is only well
known from this time onwards. By the Cambrian all major groups
(phyla) of animals, both radially and bilaterally symmetrical, had
evolved. The animals with radial symmetry consisted of the
A
8 c|
The acor worm (C), Balanoglossus spp, is « hemichordate, an intermediate
stage between the snvertebrates and the chordates ~ a group that includes the
tertebrates. The similarity between the larvae of the acorn worm ard that of
farfch (A) and sea cucumbers (B), which ave both echinadems, may indicate
the chordat’® invertebrate ancestry.
NG FORMS
COELENTRATES: ‘ECHINODERMS
Tuo forms of symmetry exc inthe invertebrate word, radial symmetry (A), in
‘ohich animale ave synmetrcal about an ais running through ther frm opto
ott, and bilateral symmetry (B), in which animale ave symmetrical about &
plane running the length of ther bodies.
coelentrates(jellyith and corals) and the echinoderms (starfish and.
sea urchins). Those with bilateral symmetry fell into four main
groups; the brachiopods ~ an almost extinct group of shellfish; the
‘molluscs — bivalved shellfish, sea snails and nautlus-like
cephalopods; the arthropods — represented primarily by the
tnilbites; and several classes of worms and worm-like creatures
From one group of these worm-like animals, the chordates, came
the first backboned animals in the Silurian ~ a class of primitive
jawwless fish and the ancestors ofall vertebrates. At this time, t00,
the plants frst came on to land. From shallow coastal waters
‘emerged a group of plants that could survive without being totaly
immersed in water. ‘They evolved stiff stems, to give therm more
support, and an internal plumbing system to transport water and
dissolved minerals up from the ground and carry manufactured
food down from the leaves.
'AS a side effect of photosynthesis free oxygen was liberated into
the atmosphere; the proportion of oxygen increased while that of
26 The similarity between lobe-fnned fish such os Eusthenopteron and early
amphibian suc as Icthyostga gives a cea indication of amphibian ancestry
Jn Icachyostga the fuks uniform spinal column has been replaced by a much
heavier and stronger stricture and a fully developed ib cage, capable of
ee ee
carbon dioxide decreased, making the compesition of air more
‘congenial to animal life. The arthropads were the frst animals to
take advantage of the improved atmospheric conditions and both
scorpions and millipedes existed among the early plants.
“The succeeding Devonian period is known asthe Age of Fishes.
First to evolve from the primitive jawless types were placoderms
such as Dinichthys ~ the armoured fishes, which had jaws evolved
from the bones of the gill arches. Before the end of the Devonian
they were largely replaced by cartilaginous fish such as Cladoslache,
the forerunners of the sharks and rays. Bony fish, more versatile and
widely distributed, existed alongside these cartilaginous species,
‘They formed two main groups ~ the ray-finned fish, which were to
prove most successful, and lebe-finned fish such as Eusthenapteron,
‘The last named is the most significant of the two from an
evolutionary point of view. Living in shallow freshwater pools
which peredially dred out gave them the evolutionary stimulus to
survive out of water. When the pools disappeared Eusthenopteron
dragged itself overland to the next area of water by means of pair
fof muscular fins evolved from stabilizing organs. At these times it
was able to breathe air through primitwe lungs developed from
‘outgrowths of the pharynx. Vertebrate life on land had begun, even
though it was only as a temporary measure to allow the
continuation of an aquatic existence. By the end of the Devonian the
amphibians, able to spend most of their adult lives on land, had
appeared. One of the earliest, Ichthyortege, showed the typical
arrangement of fve-toed limbs supported on strong girdles of bones
found in land animals. It nevertheless retained fish-lke features in
the shape of the tal and skull
‘The Carboniferous period that followed was the time ofthe great
coal forests, It was also the Age of Amphibians; the lush swamps
that characterized the lowlands of the period were ideal for their
development and consequently a large number of new forms
appeared, Some were small and eel-like, such as Dolchosoma, others
such as Exgyrinus assumed an alligator-like form and existence. Sill
others, such as Diplocaulus, became broad and flatened and lived
entirely in mud. The skulls ofthese creatures were more advanced
than the fish-like structure of Iehthyestega, The nasal passages were
well defined, indicating that they belonged to sophisticated air-
breathing animals. These animals gave rise to both the later more
highly advanced amphibians and to the reptiles
{LOBE-FINNED
FISH
Eusthenopteron
PLACODERMS:
JAWLESS FISH
AGNATHANS:
‘The carat fch were jaules (agnathar), thet mouths being no more than
openings to the digestive tack. The jaued fh first appeared in the Devonian.
‘The most primitive, the plaaxlems, were a highly dives group of armoured
species with aus and teeth formed rom bony head plates. Cartilage seletored
{fh the ancestors ofthe sharks and ays, alo appeared at this time. The bony
fch, the most mcceful group, alo descended from the agnathans, can be
ive into two clases, the lobe fone fh, which had fleshy fis, ard the ray
fined fi, hich ha ins composed of sin supported by horny fans. Most fish
‘species preent during the Age of Man belonged tothe ray-finned clas. The
lobefnned fish were represent by only four genera
27 The repiles were the first completely land/-livng vertebrate animals
on earth. The amphibians from which they had evolved were
reasonably well adapted to life on land, but always had to return to
che water to breed, and the immature stages always, of necessity
had to lead a completely aquatic tadpole existence. This meant, in
tect, that amphibian colonization of the land was confined to
| swampy areas near coasts, lakes and the banks of rivers.
HISTORY OF LIFE
THE AGE OF REPTILES
‘The reptilian development that extended this range was the
development of the hard-shelled egg, which, by means of imperme-
able membranes, enabled the embryo animal to develop in its own
private armoured pool away from water. In addition the reptiles also
had tough skins that resisted desiccation to a much greater extent
than those of the amphibians.
Although the fgst reptiles appeared among the coal forests of the
he Sees [mde
[crocoaios
irae Mammals
a |
CRETACEOUS
teen
lenthyosat
JURASSIC
Traassic
‘dinosaur
Early wre
PERMIAN
‘Mamma-ike ro
‘Stem reptiles
The carlest reptiles, own as the ‘stem reptiles, evolved frm the amphibians
in the Carborafeous and developed into variety of fos that fled all the
major ereironnental alms ir, land and water. The ichthyosaur,plesisaurs
‘and mosasaurs were aquatic, the plersaurs were aerial and the dinosaurs and
rmammal-ike reptiles tere terestrial. The dinosaurs (the terible zards) are
las in two groups aconding to thestracture of tei hips. ily, the Birds
‘are descended from the lzard-hiped group, not as one might expect rom those
with birdstike hips, AS often occurs in nature, where lick of sophistication
implies adaptability, the crocwile shape, one of the earliest reptilian forms to
evolve, ultimately proved mest sucesfl Tuo main groups of temestral reptiles evolved from reptile amphibians,
such as Seymouria ~ the archosaurs and the’ manmal-lhe reptiles, Early