Human Evolution on Trial - 'Evolution'
Like everything else, the theory of evolution is itself evolving. One hundred and fifty years ago, in Charles Darwin’s time, humans had no knowledge of genetics or the movement of the continents, and no fossils of ancient humans had been recognised. It’s only to be expected the theory has been modified since then.
Anyway Charles Darwin didn’t actually invent the theory of evolution himself. Many people before his time were convinced evolution was not just some random alternative view of history. As long ago as 1619 Lucilio Vanini believed humans might have evolved from apes (“Mythconceptions” [Ancient Myths]). And several ancient Greek philosophers had considered evolution to be a possibility. Darwin’s contribution was merely to provide evidence to support a possible mechanism for how it happened.
Of course he was influenced by the social and economic ideas of his time, the beliefs of his own social class. He was also concerned about the implications his theory had for the maintenance of the European social order, especially after the disturbances of 1848. Evolution stated very specifically there was no fixed order. This, rather than any religious doubt, seems to have been the reason he delayed publishing his theories for so many years.
Many people today still confuse “evolution” with “progress”. This confusion is a survival from Darwin’s time, the Victorian age. In those days of continuing industrial progress the concept was expanded. Progress was seen everywhere. The ultimate product of progress was believed to be “Man”, specifically English Man or, at least European Man (Jones 2001, and Tattersall and Schwartz 2000). Women didn’t come into it, which is a little strange. Any Human Evolution is impossible without them.
But until the industrial revolution all change had been seen as decline from a previous golden age. Things were better in the old days. This view too has survived. And of course the Garden of Eden was wonderful. In fact K. R. Howe (2003) suggests New Age theories on the settlement of New Zealand are based on the idea.
Biological evolution is neither progress nor decline. As the jury saw in Part II it is simply change.
Over generations the individual strands of DNA mutate or change. The causes of this change include solar or background radiation, decay with age and possibly to some extent the influence of hormones and enzymes within the body. The mutations themselves may be random (“Chromosomes and DNA” [Y-chromosome]). Pythagoras may have been onto something when he said all things, including music, are simply the product of numbers.
However evolution, unlike some music, does appear to demonstrate “purpose” or “intelligent design”. For example many scientists are beginning to accept the Gaia theory proposed by J. E. Lovelock in 1979. This states the earth behaves as if it were living even though most of it is not actually living material. Overall the earth has maintained itself in a state fit for life to exist. I read the book “Gaia” (Lovelock 1989) years ago and, naturally, it set me to wondering where humans might fit into the system. Soon afterward I went to visit one of my brothers who was working at a place called Oue on the Hokianga Harbour. His job there involved crushing the limestone that had been precipitated on the sea floor in the Mid Tertiary geological period, about thirty million years ago. The crushed limestone was then spread over farmland in order to improve its productivity. “Hmmm,” I thought.
Ultimately, though, evolution is really just the adaptation of a species to its environment. This adaptation may go back to while the individual foetus is developing; it adjusts its development to what it perceives to be the environment it is likely to grow up in. In other words before it is born an individual adapts to the environment its mother lives in. Its mother’s hormones help it do this. The adjustment may go back as far as influencing which sperm the egg accepts (“Chromosomes and DNA” [Nuclear DNA]). If all this proves to be true it could lead us back to the old idea that the giraffe grew a long neck because over the generations it was forced to reach higher and higher into the trees for its food.
In spite of prosecution claims, the overall process is most certainly not random. All species (including humans) have to survive in some sort of environment and so comparing evolution to lines of people throwing dice, thousands of monkeys typing or the construction of a watch, mousetrap or aircraft is completely absurd. The members of the defence certainly don’t believe that the eye, for example, evolved as a disembodied entity, in spite of what members of the prosecution imply. Evolution acts on whole ecosystems, not simply on each individual species in an ecosystem. And especially not on just a single characteristic of a single species. Species and ecosystems must either adapt or go extinct. From our perspective this process has led to the conception of Purpose or Intelligent Design.
In “Chromosomes and DNA” [Dominant and Recessive Genes] the jury saw that any species can be regarded as being simply a collection of shared genes, population genetics. Genes for any extreme variations are swamped by what we could call normal genes or are picked off by selection. It is not always an advantage to be the fittest and finish up in front. If a variation is not picked off, or if it provides a breeding advantage for any individuals with it, over time the whole population will change in that direction, sometimes rapidly, as the gene spreads through the generations. The bell curve shifts for that variation.
There is selection within a species, which is what is usually meant by evolution, but there is also selection between species (Tattersall and Schwartz 2000).
In the first case families become extinct; in the second species become extinct. The jury will see next in “Extinctions” [What Have we Done?] that whole genera can become extinct. Of course extinction is totally irrelevant to the individuals concerned. It makes no difference to their own life whether they leave descendants or not. It is only human culture and the knowledge of our own mortality that influences our attitude to extinctions. Contrary to most views of evolution, creatures don’t run around ensuring their genes survive. Sex and hormones are the driving forces. Genes survive as a result of a complex set of circumstances. But any species that cannot maintain effective reproductive instincts, or at least cultural strategies, obviously rapidly becomes extinct. Any species that has failed to evolve a strong instinctive will to survive in each individual is also doomed to extinction.
But the fact that DNA mutates means any populations that have become isolated from each other, for any reason, develop different patterns in their DNA, much like language change. This means populations vary from each other in different regions, and over time. Time and space again.
Evolution is the change of a species through time; speciation is the change of a species through space. The two are closely related. The space variation is usually the kind of geographic variation we saw in “The “Human Star” [Geography] and “Species” [Kinds]. But some people believe it may also occur through the exploitation of different environments by the genetic extremes of a population, ecological speciation. This is currently the main debate in evolutionary biology. I’ll explain it as quickly as I can. For ecological speciation to be possible members of a single species must be able to divide into tribes, each specialising in a different ecological niche. The jury will need to imagine that in ecological speciation the points of the star are not geographic but ecological. This means that at any one time an evolutionary star may have only two points, if such a thing could still be called a star.
It seems the greater the diversity of species in a particular region the greater the general stability of the ecosystem. But it is a chicken and egg situation. It may be that greater stability allows more species to survive. Unstable ecosystems cause extinctions.
As the defence said in “Pacific Population” the islands of the Pacific Ocean basically contain fewer and fewer biological types as you move eastwards from New Guinea towards America (Mayr and Diamond 2001). The Galapagos Islands are particularly poor in species diversity and most species there appear to derive from the other direction, America (Jones 2000). The lack of species on the Galapagos Islands makes them an interesting laboratory.
In “Change” [Galapagos Finches] the defence drew the jury’s attention to two kinds of Galapagos Island ground finches (Geospiza). They live on a single island, but can breed together in times of plenty during El Niños, and then speciate again when times are tough (Weiner 1995). This may demonstrate how separate species or kinds are able to form without being geographically separated: through ecological separation. Presumably if there were adequate numbers in each population and the stress was prolonged the kinds could remain separated long enough to become separate species. But periods of hybrid formation would make it impossible to say precisely when the two species had actually separated. Speciation would be a very gradual affair as the two populations slowly drifted apart in their ability to breed together. This shows again that we cannot assume two populations that appear in the fossil record to be different species are not able to form hybrids. Bradley Livezey (1991) even suggests that the isolating mechanisms separating various species are a result, not the cause, of factors that lead to speciation. In other words the isolating mechanisms, reproductive strategies and instincts themselves evolve or change over time as well.
Ecological speciation may be responsible for the pattern of distribution observed for many groups of species. This pattern has led to the concept of “centripetal evolution”. Tim Flannery (2001) writes, “Centripetal evolution constantly generates new species at the centre of a group’s range, leaving relictual species around the margin, often on islands”. Centripetal evolution may be an illusion though. The evolution of different subspecies in various points of a species’ star would also eventually give rise in the middle to many different genes, gene pools and even species, leaving relictual examples in other points.
Ecological speciation seems to be the best explanation for the diversification of cichlid fishes in African lakes though (quoted in Mayr and Diamond 2001) and seems to be at least reasonably common in plants. In “The First Point” [Origin] the defence will show it might have been important at times during Human Evolution.
Of course it is extremely doubtful there is actually a single means by which species evolve but the image of a star, either geographic or ecological, is useful. Gene flow from any point of a multi-pointed star usually leaves relict populations in other points. In Parts IV and V the jury will find that this situation has been very common during human development, for changes in technology, culture and language as well as for genetic change.
Most standard descriptions of evolution talk about populations somehow isolated from each other eventually becoming separate species. Possibly by this stage members of the jury can accept this could happen. I’ll return to it soon [Geographic Speciation].
But I can’t recall ever seeing a consideration of what happens when populations that have been isolated from each other long enough to change a little, through inbreeding, are able to mix again. This would be a very frequent occurrence in reality. Especially at the geographic margins of a species’ distribution during times of plenty. Or when some environmental or ecological change has occurred. Two populations that have had different selection pressures acting on them will have a different gene pool, and perhaps appearance. If the populations then meet and are able to breed together new genes are introduced into a wider population, usually through the formation of some sort of hybrid zone (“Hybrid Vigour and Inbreeding” [Wave Theory of Evolution]). In effect, though, it is actually possible for a single hybrid individual to transfer genes from one species to another (see “Hybrid Vigour and Inbreeding” [Hybrid Vigour]). Hybrid populations are usually at a disadvantage (“Species” [Ecology]) but, if for some reason a particular one is not, selection in the hybrids would promote the best combination of characteristics for survival from each of the two parent populations.
Any gene disadvantageous to survival is bred out by selection and useful genes remain. After several generations of inbreeding members of any hybrid population come to look more like each other, a stabilised hybrid. The extreme variations are eliminated. Of course some disadvantageous mutations may survive if the individuals that carry the single recessive receive enough heterosis to be at a selective advantage.
If there has been inbreeding depression in either original population the resulting hybrid population may also display hybrid vigour and displace one or both parent populations. For example, like all creatures, the duck genus Oxyura is divided into various species and subspecies throughout its range. The white-headed stiff-tail (Oxyura leucocephala) occupies the Western Mediterranean, Central Asia and Northern India, but its range is discontinuous. The Spainish population has become isolated and presumably inbred. The North American ruddy duck (Oxyura jamaicensis) has been introduced to Spain and within that country this introduced population probably also constituted an inbred population. Since the introduction hybrids of the two species have formed, and seem to be replacing the native white-headed duck by gene flow between the two species (Tudge 1996). It seems that eventually the population in Spain will evolve into a stabilised hybrid. As the defence said in “Species” the formation of hybrids may have been very important during duck evolution.
A similar process probably happens, at least at a minor level, for all species at all times.
Any new species can be only a little different to its parent species. But any new species may have a major initial expansion if it is able to move into a new region or use the environment in a new way. We may be able to get some idea of what happens then by observing species that have been introduced to new environments by humans. The jury saw in “Change” [Destruction] that even human numbers usually increase to epidemic proportions. It seems that the depletion, or even local extinction, of resources follows an initial population explosion during the first times of plenty. After the initial population explosion numbers fall with the onset of hard times. Population remnants survive in areas where the environment can support them in some sort of balance. These scattered population remnants become inbred. They either slowly change, each in their own direction, or the species becomes locally extinct.
Populations that have been able to evolve long-term survival strategies can then expand back through what has become an unoccupied ecological niche. Eventually some of these groups may be, for one reason or another, able to come in contact with other inbred populations. They either form a hybrid or are already separate species. But if two or more of the populations are able to interbreed, selection can start on the hybrid population and we’re off again.
As the jury saw in Part II, something like this happened as humans moved onto islands in the Pacific. It has presumably happened at other times during human development, as populations have been able to expand into new environments.
The process is probably responsible for what appears in the fossil record to be the relatively rapid speciation of any creature that has been able to invade a new environment. At a greater level it would give the impression in the fossil record of fairly rapid change or the sudden appearance of new kinds, which is often exactly what we find.
It is becoming generally accepted that the fossil record shows that evolution usually proceeds by a series of jumps separated by long periods of very little change. Evolution is digital not analogue. The phenomenon even appears at a very local level. For example Corfield (2001) describes the sudden bursts of change in species of snails around a lake in Kenya. In 1972 the late Steven J. Gould along with Niles Eldredge introduced the term “punctuated equilibrium” to describe the phenomenon.
Most species stay roughly the same for a long time, because they are well adapted to an environment that doesn’t change (Tudge 1996). For example sharks and crocodiles have remained much the same for a very long time, although many varieties have become extinct. Another reason for stability is that mutations that actually increase survivability are probably quite rare, and the formation of a double recessive even more so. In “Hybrid Vigour and Inbreeding” [Wave Theory of Evolution] the defence explained how an advantageous gene might rapidly expand though.
And sometimes changes in the earth’s environment, either through changes in weather patterns or even the evolution of some new species, have led to very rapid evolutionary change. Even apparently minor changes can have a huge effect. Eventually everything settles down a bit and we have a period of stability. These factors all lead to the phenomenon of punctuated equilibrium. But in fact the balance of nature is constantly changing.
Many people wish to convince us that a huge leap or series of leaps has occurred at some stage during Human Evolution, either between animal and human or early human and modern human. Later we’ll stand back and be able to take an overall view of the evidence. The jury will then see that our evolution has basically proceeded by gradual change at a fairly leisurely pace (a number of small jumps) rather than by the few sudden huge jumps people look for in punctuated equilibrium. In other words we may be looking at a punctuation mark in the process of evolution. Its study may give us a good idea of how these punctuations work.
The defence pointed out in “Hybrid Vigour and Inbreeding” that the geographic margins of a population are the most different. Unless a population’s members are extremely mobile and genes can travel easily from one end to the other, gene combinations (or at least mating behaviours) from opposite ends of a widely spread species may eventually become incompatible. Conveniently for the defence case the geographical range of several collections of subspecies forms a circle, and the opposite ends of their distribution overlap.
Both the herring gull (Larus argentatus) and the lesser black-backed gull (Larus fuscus) are found in Northwest Europe. But as you follow the black-backed gull eastwards across Northern Asia the black on its back fades gradually to slate grey. The herring gull’s silvery grey back, on the other hand, becomes darker as you move west across the Atlantic and through North America to Northern Asia. The two species are simply opposite ends of a cline that stretches around the Arctic. But the herring gull and the lesser black-backed gull don’t form hybrids where the ends overlap. They are recognised as being separate species.
Rory Putman (1988) writes, “A parallel situation exists in Eurasian great tits, Parus major, where a series of populations extends from the east coast of Soviet Russia, through Russia itself to western Europe, thence south of the Caspian Sea, through to India and on northwards through China and Korea back to the eastern coast of Russia. While the various populations around this circuit differ from one another, they intergrade, and it is clear that adjacent, neighbouring populations regularly hybridise. However, by the time the circle is complete, where the two ends of this continuous ring meet at the Amur River in USSR they have diverged sufficiently that they themselves cannot interbreed, despite the fact that they intergrade with each other through neighbouring populations right around the ring and so strictly might be considered one biological species”.
Putman also suggests the separate species of deer in the genus Cervus (which includes red deer, wapiti, sika and sambar) are also just variations of a single species. Geography has restricted gene flow between the separate species. Red deer, at least, are capable of forming fertile hybrids with most of the others.
The defence has pointed out many times that new gene combinations move through populations by the formation of hybrid zones, the wave theory of evolution. If the populations are closely related the hybrid zone can be wide and we have a cline. If the populations are less closely related hybrid zones are narrower. The difference may even be so great hybrid zones are unable to form. The spread of any new gene combination is then achieved by the expansion of what has become a new species.
It seems that many species originally occupying a wide geographical range have developed into different species in different regions. These separate species have then spread back over much of the original species’ range, separating the ecological niches. Each of these species has diversified yet again. In other words the process is continuous, much like language movement and change. As James Hutton said (“Long Ago” [Geology]), “We can find no vestige of a beginning, no prospect of an end”.
The ducks we met in “Species” [Kinds] certainly seem to have separated from each other through geographic speciation, as may have cats, hyenas, bears and dogs. In fact antelope, llamas, camels, giraffes, deer, sheep, goats, cattle and pronghorn antelope can be arranged to demonstrate the process perfectly.
Future detailed examination of nuclear and mitochondrial DNA and the Y-chromosome will no doubt lead to some adjustments to the following diagram.
And of course there is actually much greater diversity than shown. For example there are a huge number of species of deer, goats, antelope etc. and there were even several subspecies of American bison when Europeans first arrived there.
Some sort of time scale can be guessed at for the diagram.
First of all camels and llamas are not true ruminants as they have only a three-stage digestive system rather than four (Tudge 1996). Camels, like horses, had their greatest diversity in North America and didn’t appear in Asia until about four million years ago (Flannery 2001). But camels had started developing more than 40 million years ago from pig-like ancestors. And. Grassland expanded around 25 million years ago and other evidence shows the deer and giraffe line had separated from the main line by then (Tudge 1996). Deer and giraffes remained animals of the forest or forest edges (Putman 1988) while the other ruminants took advantage of the grassland. Deer probably developed in Eastern Asia about 20 million years ago. Giraffes are now confined to Africa. About nineteen million years ago the ancestors of the pronghorns and their extinct relations moved into North America from Asia. There is now only one species in this group but during the Miocene geological epoch they had diversified into a very weird collection (Flannery 2001). Deer first appeared in North America five million years ago although one group (the “hollow-toothed deer”) is virtually confined to America, especially South America.
Until humans arrived there myotragus lived on the island of Majorca. It could be described as a sort of antelope (Attenborough 1987). It had probably been isolated on the island since the filling of the Mediterranean Sea, three to five million years ago. The split that led to antelope, goat, cow and sheep must have occurred before then. Gazelles first appeared about 2.6 million years ago (Tudge 1996) and have their greatest diversity in Africa. It’s possible that in Asia early humans exterminated them though.
Research by Buntjer et al (2002) has revealed a great deal of information for the last few branches in the diagram. Certainly all the species from bison on developed simply as regional and ecological varieties of a single original species. The diversification began as recently as one million years ago. The relationship between the species is not simple however. They do not simply split immediately into the separate species. Gene flow between the various species has occurred at various times. The most interesting case is the bison. All bison species look roughly the same; their nuclear DNA is similar. But the European bison’s mitochondrial DNA is much closer to that of cattle than to that of American bison. The mtDNA and nuclear DNA lines are surprisingly independent. Presumably many other genes are also independent. The defence asks the jury to remember this.
The defence mentioned Indian, African and European cattle mtDNA in “Pedigrees”(Bradley et al 1996). African and European cattle separated 22,000 to 26,000 years ago. [Selection]. The three groups of cattle can all easily form fertile hybrids. They can even form hybrids with bison. Indian cattle split from African and European cattle 117,000 to 275,000 years ago
The Human Influence
Until recently domestic cattle were bred for dual or triple purpose: milk, meat and pulling wagons or ploughs, but the development of the internal combustion engine has released cattle in the Western World from draught purposes. And within the last 100 or 200 years European cattle have been further split into dairy and beef breeds.
This splitting of European cattle into dairy and beef breeds fits onto the above diagram absolutely smoothly. Any conception it might represent something new is completely wrong. Humans are as much a part of the selection process as are any group of lions on the African plains or any climatic events such as El Niños in the Galapagos Islands. It is remotely possible some form of ancient human activity led to the split between cattle and bison in the first place, and in “Extinctions” [The Results] the defence will suggest it is even possible that human activity had earlier encouraged the diversity of antelope and gazelle species in Africa.
Because it provides us with interesting evidence in favour of the defendant we’ll study animal extinction during human expansion next. One difficulty arises in trying to decide whether a species has in fact become extinct, or if it has changed into something else. For example the long-horned bison disappeared in North America soon after humans arrived. But a lot of evidence shows it actually changed by selection through human hunting and landscape alteration (Flannery 2001). It probably also formed a series of stabilised hybrids with the incoming Asian bison and became the modern American plains bison (Tudge 1996). This sort of change didn’t happen with all the species in North America. Mammoths and mastodons, for example, obviously became extinct.
Dogs have also changed and diversified considerably since they were first domesticated, probably a little more than 10,000 years ago. In fact if we were reduced to just examining their skeletons many breeds would no doubt be classified as separate species. Of course size differences do prevent successful mating between some pairs of breeds. And Steve Jones (2000) claims, “Even when large forms like Great Dane and St Bernard are mated, the young are defective, as they inherit so many genes for abnormal growth”. Does this mean we can now call them separate species? But most dog breeds can successfully form fertile hybrids, even with wolves.
Human activity has certainly had a hand in the evolution of plants.
For example many new hybrids of previously geographically isolated plants have been made in the last 300 years, often in plant nurseries. Some of these hybrids, such as ragwort (Jones 2000), blackberry, rhododendron, lantana, old man’s beard and spartina, have become noxious weeds in many countries (although one of my friends says concrete is actually the worst noxious weed).
But the change in crop plants actually began more than 10,000 years ago. Once we have any form of domestication the direction of selection changes. Studies have shown grains such as wheat and maize evolved interesting changes as a result of the prehistoric human harvesting, and eventually sowing, of wild forms. These changes were mainly in the size of the individual grains and in the ease of harvesting. The first step in the domestication of grasses would have been the carrying of wild grass seed-heads back to a seasonal home base. Those strains of grasses that held the seeds more firmly to the head by a strong rachis would have been the ones that made it there (Attenborough 1987). By the next season the grains that had been spilled around the campsite would have grown. They would then have been the easiest ones harvested. Natural selection for a strong rachis. The process would have been gradual though, almost certainly over a period of several thousand years.
Collecting of grass seed may have started in the Middle East as long ago as 50,000 years, in Neanderthal times (Jones 2001). Once humans discovered sowing the seeds was possible selection for large seeds could have been rapid. This would have been helped by the survival of multiple-chromosome grasses formed through mutations (e.g. tetraploid). The mingling of different strains by trading would also have given rise to hybrids through cross pollination (Jobling et al 2004), a similar process to what has happened with the nursery hybrids formed in more recent times.
The jury will soon see that all the above patterns continue throughout Human Evolution. But the defence has one last piece of evidence to present before we can begin to follow our evolution from apes until today.
See next :: Human Evolution On Trial - 'Extinctions'
Attenborough, David (1987) The First Eden. Guild, London.
Bradley et al (1996) Mitochondrial Diversity and the Origins of African and European Cattle. Proc. Natl. Acad. Sci. Vol. 93 pp. 5131-5135.
Buntjer et al (2002) Phylogeny of Bovine Species Based on AFLP Fingerprinting. Heredity 88, 46-51.
Howe, K. R. (2003) The Quest for Origins. Penguin, New Zealand
Jones, Martin (2001) The Molecule Hunt. The Penguin Press, London.
Livezey, Bradley C. (1991) A Phylogenetic Analysis and Classification of Recent Dabbling Ducks (Tribe Anatini) Based on Comparative Morphology. The Auk, 108: 471-507.
Lovelock, J. E. (1989) Gaia, a New Look at Life on Earth. Oxford University Press, Oxford.
Weiner, Jonathan (1995) The Beak of the Finch. Random House, London.