34

2 Endless forms most beautiful

There is grandeur in this view of life, with its several powers, having been

originally breathed into a few forms or into one; and that, whilst this

planet has gone cycling on according to the fixed law of gravity, from so

simple a beginning endless forms most beautiful and most wonderful

have been, and are being, evolved.

Charles Darwin Origin of Species

If you were to journey back 500-odd million years to the Cambrian and have a walkabout, the familiar landscape would contain few signs of life. On the conti-

nents at least, the world had not changed much in the over 3 billion years since life first emerged. The continents equivalent in size to present-day Africa, South America, Australia, Antarctica and India had collided together forming Gondwana, a super-continent that spanned from the South Pole to the equator. Ice sheets came and went episodically over the pole, but the major icing-overs of snowball Earth were long past and never to return. Global climate was on a warming trend and the continents were fringed by vast shallow seas. It was in these shallow seas rather than on land that the many animals of the Cambrian explosion thrived initially.

You would have had to leave the shore and snorkel or scuba-dive in order to see the many new complex life forms – some fixed to the seabed, others floating or swim-ming about. Many varieties of red and green algae had diversified in abundance, but brown algae, and more significantly complex plants, had yet to evolve. Many algae

35

were still single-celled plankton, while others formed simple colonies or more com-plex multicellular structures with leaf-like fronds rising up from holdfasts fixed to the seabed. You might see the odd stromatolite dome, but the algae had largely displaced the cyanobacteria at the base of the food chain. The water column was full of living and dead bits of algae, which were being filtered out and consumed by a variety of long-existent sponges. Also present were the survivors among the Ediacaran animals, mostly jellyfish and simple corals. What was strikingly new about Cambrian seas was their diverse community of animals. Some scavenged food as they crawled along the seabed; some devoured others while darting about in the water column. These were the first of the bilaterian animals, which had previously featured in only a minor way, but now came into their own.

Among the newcomers were some with unusual body shapes and structures: the two-metre-long Anomalacarid, the multiple-eyed Opabinia with its long feeding appendage, and Hallucigenia walking about on tube-like legs

and covered in protective spikes. Descendants of the Ediacaran bilaterian fossil Kimberella may have

evolved into the first shellfish (brachiopods and molluscs) and furrowing Ediacaran bilaterian organ-isms may have evolved into the first worms. However, the origin of most Cambrian animals remains obscure.

Living in Cambrian seas,

along with existing sponges

and jellyfish, were many

new animals, including the

large anomalacarid, annelid

worms (and other burrowing

organisms such as Ottoia

front right), the crawling

spiked Hallucigenia (lower

left) and the swimming

Marella and Pikaia (upper

left) - possibly our earliest

fish-like ancestor

Hallucigenia Opabinia

36

HUMAN ORIGINS

What we do know is that 99% of all animal groups living today made their debut in the fossil record during the Cambrian explosion.

The Cambrian explosionThe many new Cambrian animals didn’t all arrive at once: most took time to evolve and continued to diversify throughout the Cambrian. So the Cambrian was not as ‘explosive’ as originally thought, but unfolded over the 70 million years commencing near the boundary at 541 million years ago and continuing into the early Ordovician period up until around 470 million years ago. Nevertheless, it was a period of major evolutionary innovation relative to the billions of years leading up to it. It was when all the existing basic body plans of bilaterian animals first appear in the fossil record.

Similar to a floor plan of a building, a body plan is how the parts of an animal are organised and structured. All living and fossil animals can be assigned to a group, called a phylum, on the basis of their body plan. Biologists define a total of 30-odd different phyla into which the animals of the animal kingdom can be placed. We belong to the phylum Chordata, and along with all other animals possessing a back-bone we belong to the chordate subphylum of vertebrates. Not all chordates have a

Protozoa (choanoflagellates)

510

541

PROT

EROZ

OIC

diver

sifica

tion

millio

ns of

year

s ago

Ediac

aran

??

cnidarians

750

600

? ?

?

CENOZOIC

MES

OZOI

CPA

LEOZ

OIC

0

66

252

snow

ball E

arth

episo

des

?

?

Arth

ropo

da

Moll

usca

Echin

oder

mata

Chor

data

Cnida

ria

Brac

hiopo

da

Porif

era

Anne

lida

Cambrian‘explosion’

Centophora (comb jellies)

Placozoa

spon

ges

bilaterians

Nearly all the major groups (phyla) of animals (some shown

here) first appear as fossils in the Cambrian explosion

37

Endless forms most beautiful

backbone but they all have, at some stage of development, a notochord – the cartilaginous rod that runs down their back associated with nerve fibre bundles.

The phylum Arthropoda, to which the extinct trilobites and all living insects belong, is defined by segmented bodies having paired jointed limbs and an external hard cover (exoskeleton). The phylum Echinodermata includes sea stars and sea urchins, and is defined by its distinctive five-fold (pentaradial) symmetry. The phylum Brachiopoda includes marine shelled organisms (lamp shells) whose upper and lower shells are different in size or shape but each is bilaterally symmetric. In contrast, the line of symmetry defines identi-cal left and right shells belonging to bivalves (clams and mussels) in the phylum Mollusca. These are just a few examples of the many distinct body plans that evolved during the Cambrian explosion.

The interpretation of the many early animals of the Cambrian explosion has varied over the years, but it is now thought that as bizarre as some may look to us today they can all be assigned to existing body plans. Although these basic animal body plans have not changed – no phylum has become extinct and no new phylum has appeared – there have been many evolu-tionary innovations within each phylum since the Cambrian. It is these variations, the amazingly diverse forms of life that have evolved since the Cambrian, that Darwin’s ‘endless forms most beautiful’ refers. Each phylum rapidly gave rise to its own finely branching tree of life as the DNA within each was passed on in modified form with the evolution of new species. The animals of the various trees co-evolved as they interacted and influenced one other to varying degrees, but the direct exchange of genes was restricted to closely related species. Some interactions included conflict as individuals competed against members of their own and other species for limited resources in an ever-escalating arms race between predator and prey. Other interactions included cooperation as species developed mutual-ly beneficial relationships (symbiosis). Species came and species went: in fact, an estimated 99.9% of all the species that ever lived have become extinct. But all the estimated 5 to 10 million species living today, including us, owe their existence to a long chain of descent by way of many ancient, now extinct, ancestors.

Sea star

Fossil lamp shell

Black mussel closed (top) and opened (bottom)

Trilobite

38

HUMAN ORIGINS

Our descent by way of all those in our phylum who came before us can be traced from the fossil record. However, constructing any tree of descent is complicated by the fact that only a minute fraction of all the organisms that ever lived end up preserved as fossils, and fossils preserve only a fraction of what were once-living organisms. This incompleteness of the fossil record results in many gaps, which make it difficult to trace lines of descent. Also, some features are lost over time and other features that appear similar are not necessarily related by descent. Similar features can come about by convergence, where two different and unrelated animals happen to evolve a similar feature independently. Eyes and wings, for example, evolved multiple times within different and unrelated groups.

Despite these complications and limitations, enough fossils have been found to give us at least a broad understanding of how animal life has evolved through time. Our descent can be traced from the earliest ribbon-like creatures having a notochord to the many different types of fish, one group of which would give rise to the amphib-ians, who ventured onto land and would eventually evolve into reptiles and mammals. The mammals were a diverse but small-bodied group until the dinosaurs exited the scene –after which larger mammals evolved, including monkeys and apes, our closest primate ancestors.

How the many different animals of each branching tree came about, including our descent among vertebrates is best explained by Evolutionary Theory. Contemporary Evolutionary Theory is built upon Charles Darwin’s theory of evolution by natural se-lection. Although many are familiar with evolutionary concepts, it is worth reviewing Darwin’s proposed mechanism of natural selection and Evolutionary Theory in gener-al before tracing our evolutionary pathway of descent over time. Of course, everything about the evolution of life discussed up to this point can also be understood in terms of Evolutionary Theory.

Convergence: wings for flying have independently evolved in unrelated groups including insects, fish, birds and bats

39

Endless forms most beautiful

Darwin’s theory Darwin’s book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1859) is the most elegant and lasting explanation of how life evolved on Earth. Darwin based his theory on several key observations. First, many more individuals of a species are produced each generation than survive long enough to contribute to the next (people living in the developed world are a recent exception). Second, each individual, although having many features in common with other members of its species, is unique, and the many individuals present a wide range of variations in any given generation. Third, many of these variations are heritable, and for those individuals who produce offspring, some of their unique variations are passed on to the next generation. From these observations, Darwin proposed that organisms evolved by way of natural selection.

Natural selection is the process by which heritable traits become more or less fre-quent in a population as a result of the relative reproductive success or ‘fitness’ of in-dividuals. Reproductive fitness refers to how successful individuals are in transferring their genes to the next generation. Not only must individuals survive long enough to reach sexual maturity, they must find a mate and they must produce offspring. De-pending on whether those genetic traits they pass on enhance the survival and repro-ductive success of their offspring will largely determine if their genes are likely to be carried forward in future generations. Traits can be selected for or against depending on whether they are beneficial or deleterious to those who possess them within the habitat in which they live. Habitat is defined by all aspects of the physical and biolog-ical environment in which an organism lives and includes interactions with other life forms, especially members of the same species.

Natural selection produces adaptations that make an organism better able to survive and reproduce in its habitat. However, in some cases a trait adapted for one function can become advantageous for other functions (exaptation), whereas other traits may become obsolete (vestigial structures). Natural selection doesn’t strive to produce a perfect fit of organism to habitat. It is far more

Our vestigial tail (coccyx) bone

Charles Darwin (1840)

40

HUMAN ORIGINS

pragmatic than that, as it must be, confined to working with the variations on offer in any given generation. The result is many less-than-ideal trade-offs, but ones which have produced, on average over evolutionary time, organisms that are well enough adapted to persist generation after generation in highly diverse and changing habitats.

Natural selection refers to those forces of nature that act to select individuals best adapted to their habitat. But no intention is implied in the process. Nature isn’t con-sciously selecting the best to survive in the same way we might consciously select the largest apples to cultivate. Our purposeful selection that transformed 2.5 cm (1

in) wild apples into large cultivated apples is called artificial selection. Darwin used artificial selection as an example of how selection can lead to specific traits, such as the hun-dreds of distinct features produced by fancy-pigeon breeders. Artificial selection is usually carried out with a specific purpose or goal in mind. But natural selection has no purpose or goal other than that those organ-

isms having relatively greater fitness are more likely to survive and to reproduce. Nor does natural selection have anything to do with our notion of progress; there is often no obvious reason (to us) why some species become extinct while others do not.

Another common misconception is that natural selection is an entirely random process in which only the lucky survive by chance. Although natural selection operates on variations among individuals that are randomly produced by rare copying errors (mutations) in the replication of DNA or in the random mixing of their parents’ DNA, natural selection itself is not a random process. It will depend on the specific traits of an organism in relation to its habitat. And because natural selection cannot anticipate or plan for future events, what was advantageous for one generation may or may not be for the next. Each passing generation is subject to a unique combina-tion of selective forces out of which those having physical and behavioural traits that favour their reproductive fitness carry on. In this way species can change fluidly with each generation, selectively sculpted to best fit their habitat. There are different types of natural selection as well as levels at which it can operate (individual and group), and these are discussed in Chapter 5 in relation to how new species come about.

Fancy pigeons

41

Endless forms most beautiful

Although an important process in evolution, natural selection is not the only mechanism that can cause the frequency of genes in a population to change. When selective forces are absent or weak, it is possible for the genetic composition of a pop-ulation to drift randomly. This process, referred to as genetic drift, tends to occur most readily in small populations or when one population is split into two isolated popula-tions that then drift apart. It is also possible for a single random genetic mutation to have a significant evolutionary impact. However, most mutations are detrimental and quickly die out or they are neutral and have no significant impact. As we shall see, sin-gle mutations and genetic drift have both been shown to be important in the evolution of some human traits, but in many cases it is difficult to determine the relative role of mutations, genetic drift, natural selec-tion and other mechanisms of evolutionary change.

Whatever the mechanisms involved, the fossil record reveals that overall species diversity has increased from the Cambrian to the present day. This increase is gen-erally interpreted to reflect expansion of organisms into new and variable habitats, co-evolution (predator-prey conflict and cooperative symbiosis) and a move away from generalists to increasingly more specialised species. The record is consistent with Darwin’s ‘modification by descent’ in which tweaking what already exists is generally more likely to be successful than are radically new structures or freaks of nature. Such modifications can allow for initially simple structures, such as a light-sensitive cell of a jellyfish, to evolve into more complex ones, such as the human eye, over time. And although most species become extinct, their DNA (genome) in modified form carries on in their descendants. Darwin emphasised gradual, step-by-step changes in evolu-tion, while others view evolution as long periods of relatively few changes punctuated by cascades of rapid change. The fossil record reveals that the increase in diversity was interrupted by episodic catastrophic mass-extinctions events during which large num-bers of species suddenly became extinct. As we shall see, the ‘big 5’ mass extinctions since the Cambrian played a significant role in our evolutionary descent. Because evolution cannot anticipate or plan for catastrophic events, these act episodically to

Orig

inal

pop

ulat

ion

isolated population A

isolated population B

multiple generations

multiple generations

Barrier

Genetic drift: changes in the genetic make-up of isolated populations (usually small)

that can occur over multiple generations by random events

42

HUMAN ORIGINS

wipe out much of the living landscape. The emptying of many species from previously full habitats makes room for the survivors to expand into. Such opportunistic diver-sifications, or adaptive radiations, typically follow mass extinctions and can result in the relatively rapid appearance of many new species.

The cause of most of these massive die-offs remains debated and many appear to have multiple factors involved. However, the principal culprits in mass extinctions include major global climate change, often in association with intense volcanic erup-tions or meteorite impacts. The ability of a species to survive any change, including catastrophic ones, depends on a number of factors, not least of which is just plain luck. But it is not just the lucky who survive. The more individuals there are, and the

more widely distributed they are, the greater a species’ chances of survival. Size also matters, with large life forms particularly vulnerable to extinction when there is an interruption in their food supply. Finally, a more diverse genetic makeup enhances the probability that at least some individuals will have features that allow them to survive and reproduce. Thus, large dinosaurs are more vulnerable than small mammals, which are in turn more vulnerable than bacteria to extinction.

Much has happened in our understanding of evolution since Darwin, but Darwin’s observations and his theory of natural selection to explain them still hold. The biggest advancement has been in our understanding of the role DNA plays in producing vari-ations and how these are transferred from one generation to the next. The mapping of

541 500 450 400 350 300 250 200 150 100 50 0

0

1000

2000

3000

4000

5000

Cam Ord Sil Dev Carb Per Tri Jur Cret Pal Neo

The number of major marine fossil animal groups since the Cambrian explosion, with major reductions associated

with the ‘big 5’ mass extinctions

End

Cret

aceo

us

End

Tria

ssic

End

Perm

ian

End

Ordo

vicia

n

Late

Dev

onia

n

43

Endless forms most beautiful

the entire human genome was a major accomplishment, but the reading of our DNA blueprint in terms of who we are is far from understood. One surprise was how few genes our DNA codes for: only around 21 thousand, and these produce on the order of 100 thousand proteins. Rather than a straightforward connection of a particular gene to a particular trait or attribute, most of who we are and how we function in-volves extremely complex interactions of many genes and the proteins they produce. In addition, the highly convoluted interactive pathways of the proteins expressed by our genes can be influenced by changes in our bodies as well as our environment.

Only around 2% of our DNA consists of genes that code for proteins. The other 98% was originally considered to be ‘junk’ DNA. Our DNA undoubtedly includes a hodgepodge of disused remnants, duplications and viral parasites accumulated throughout its long history. Many of these may no longer be active; however, it is now clear that some junk DNA determines which genes do get expressed (turned on or off) and when. Some claim that as much as 80% of junk DNA plays a role, with potentially hundreds of thousands of so-called enhancer regions that regulate gene expression throughout the genome. To further complicate matters, heritable traits can be produced by processes not related to changes in the sequence of DNA base pairs. These processes are referred to as epigenetic, reflecting the fact that they are over and above and hence outside of the DNA molecule. Epigenetics works through chemical changes surrounding the DNA molecule, which can alter how and when a particular gene is expressed without altering the DNA sequence. Epigenetic changes may be associated with particular stages in human development or to changes in the environment, and may be directly passed on from parent to child. Therefore, knowing the DNA code or base pair sequence is only a start: DNA is not simply a static code for life, but a highly dynamic molecule, constantly contorting its tightly-tangled self and its gene expressions in response to environmental cues.

Another surprise is how similar our DNA is to the DNA of other organisms: we share 90% of our genes with mice, 50% with bananas and around 15% with bacteria. The DNA of animals is highly conserved, meaning that all animals share many simi-lar genes. This is consistent with the idea that all multicellular animals derive from a single common ancestor, as discussed in Chapter 1. The concept that ‘all life is one’ may seem hard to accept given the incredible variation evident in all past and present life forms. However, these variations in life forms demonstrate the versatility of DNA and how relatively few changes in the genetic sequence (genotype) can result in major differences in body forms (phenotype).

44

HUMAN ORIGINS

One reason that two very different-looking animals can have very similar DNA is attributed to variable expression of the same master genes, called Hox genes. Hox genes are regulatory genes that play a fundamental role in defining the various cell types used to build the different animal body types. They control the body plan of an embryo by calling up particular structural genes at specific times during growth and development. Simple sponges have relatively few Hox genes compared to the fish-like lancelet and humans. These differences suggest that much of the increase in animal complexity from a sponge to a human is related, in part, to the copying of an ever-greater variety of Hox genes to regulate the production of the many diverse cell types that are needed.

So, although Evolutionary Theory is well supported by observations, its detailed workings remain an area of active research, and new findings are likely to change our understanding of evolutionary processes and our interpretation of past evolutionary events. This state of affairs should not be construed to indicate that there is something ‘wrong’ with Evolutionary Theory; rather it reflects the nature of scientific inquiry. Invariably the longer and more intensely we observe our world, the more its complex-ities are revealed. These complexities in turn allow us to refine our understanding of how evolution works. In that respect, Darwin’s theory of evolution by natural selec-tion, like all scientific theories, has itself evolved and will continue to evolve with new knowledge, methods and perspectives. These are the dynamics of science that make it so exciting. Let’s now see how evolution shaped our vertebrate tree of life from the earliest fish-like animals living in the Cambrian until 500 million years on to the evo-lution of the great apes, the family to which we belong.

Sponge Lancelet Human embryo Human adultHuman

Animals have similar

Hox genes (coloured

boxes) but the number

of Hox genes varies

among animals as

shown here for

sponges, the lancelet

(amphioxus) and

humans

45

Endless forms most beautiful

Your inner fish In our long evolutionary journey since the Cambrian we started out as fish, or fish-like animals, swimming about in the sea. And in our phylum fish remain a highly successful group, with their estimated 31 thousand species accounting for half of the known living vertebrates. While hypothetically snorkelling in Cambrian seas, you might spot our earliest fish-like ancestors, darting about and desperately trying to avoid being eaten by an anomalacarid. As with most animal phyla that first appeared in the Cambrian explosion, the origin of our earliest members is difficult to decipher. In part this is because they initially lacked hard mineralised skeletons.

Unlike the hard chitinous carapace of trilobites and the hard carbonate shell of brachiopods, most of our early ancestors had bodies, including their notochords, too soft to be easily preserved. It is only in exceptional deposits that we find any trace of what they looked like, squashed between layers of fine mud. In fact, most of what we know about the earliest Cambrian animals comes from just two such rare deposits having exceptional fossil preservation – the 525-million-year-old Chengjiang deposit in China and the 505-million-year-old Burgess Shale in Canada (and upon which the diorama at the beginning of this chapter is largely based).

Myllokunmingiia fengjaoa

fossil

reconstruction reconstruction

fossil

Burgess ShaleChengjiang

Metaspringia walcotti Pakaia gracilens

amphioxus (lancelet)

reconstruction

dorsal fin

mouth

gill pouches (?)intestines

ventro-lateral fin

notochord (?)myomeremuscle blocks

mouth

intestines

notochord

gill slits caudal fin

dorsal finmyomeremuscle blocks

nerve chord

tentacles

fossilfossil

eyes

Early chordates include exceptional soft-body fossils from Chengjiang in south China and the Burgess Shale in

Canada. Myllokunmingids were similar to the living lancelets (amphioxus)