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problems for darwin : the step to human, lecture one

Chimpanzees are the species most closely related to humans, and it seems that that speciation, from a common ancestor, occurred perhaps 6 million years ago.

The above paragraph does not say that chimpanzees, or humans, walked this Earth six million years ago. It says that our last common ancestor walked this Earth then and that at some later time speciation occurred. One species evolved into chimpanzees. The other evolved into humans.

The evolutionary line that has evolved into humans is called the hominid line.

Our next closest relations are the other great apes, bonobo, gorilla, and orangutan. Good guesses are that our (and chimpanzees and bonobo) last common ancestor with gorilla lived about 7 million years ago and that our (and chimpanzee and bonobo and gorilla) last common ancestor with orangutan lived about 14 million years ago.

This lecture and the next look at two challenges to Darwin’s theory of evolution by natural selection posed by the evolution of humans over the last 10-14 million years.

• 1. Both the human and chimpanzee genomes have been sequenced. Has there been enough time for humans to have evolved by natural selection from a common ancestor with chimpanzees?
• 2. The great apes are gorillas, chimpanzees, bonobos, orangutans and humans. Orangutan are found in Asia, gorilla in Africa. Is it possible that hominids evolved independently in Asia and Africa and the two sub-populations began to interbreed when hominids emigrated from Africa?

This second point would be a catastrophe for Darwin’s theory of evolution by natural selection. It is a strong version of the multi-regional hypothesis and it proposes that Homo sapiens evolved independently in several areas of the world at roughly the same time in evolutionary terms.

It would be a catastrophe for Darwin’s theory of evolution by natural selection because according to that theory speciation in Asia from a common ancestor with the orangutan would result from accumulated random mutations. It would be too incredible to believe that accumulated random mutations in Africa from a common ancestor with gorillas would produce the same species as that arising from accumulated mutations from a common ancestor with orangutans in Asia.

Has there been enough time for humans to have evolved from a common ancestor with chimpanzees?

Humans differ from other apes in, among others:

• the development of speech (not evidenced in the fossil record)
• the development of a bigger brain and flatter face (no muzzle), without the prominent ridges,in a similar sized skull
• smaller canines and a parabolic rather then rectangular dental arch
• upright gait, evidenced by hip joints which direct the femurs so the knees meet in the midline. Other apes are bow-legged (knees are 'outside' the hip rather than 'inside' it. The ilium is shorter in humans.
• upright gait is also evidenced by the exit of the spinal cord (the foramen magnum) from the bottom of the skull in humans and the back of the skull in other great apes
• The adoption of an upright gait has led to anatomical changes in the human foot. For example, unlike other primates humans do not have an opposable big toe (the human big toe can not touch all the other toes)

To consider whether there has been enough time for humans to have evolved from their common ancestor with chimpanzees, it is necessary to make a few assumptions.

Although chimpanzees reach sexual maturity at about age seven, a female chimp does not become a mother before age 13-14.

For the sake of argument, let us say that humans diverged from chimpanzees about six million years ago, and that the average generation time for the hominid line since has been 15 years, so that 400,000 generations separate us from our last common ancestor with the chimpanzees.

Jerry Coyne in Why Evolution is True reports that of the approximately 25,000 genes in our genome, there are over 1,400 genes that chimpanzees don’t have. We have different numbers of copies of some genes that we possess in common with chimpanzees, and there is a difference in the amino acid sequences of about 20,000 of the genes we have in common.

Can Darwin’s theory of evolution by natural selection assisted by mutations and crossovers and so on explain the changes that humans have undergone in that time period?

We are unlikely to ever know the genome of the last common ancestor from which both species have diverged. However, it does seem like a formidable amount of change to have occurred in a small period of evolutionary time.

The estimate of 400,000 generations is also too large. The time available is the time between when the hominid line diverged from the chimp line (i.e the time when early hominids could no longer breed with the descendants of the 'chimp' line) and, looking backwards from now, the time when modern humans could not breed with our hominid ancestors.

Chimpanzees (and the other great apes) have 24 pairs of chromosomes. Humans have 23 (two of the chimpanzee chromosomes have fused to become human chromosome 2)

I have no idea if any attempt has been made to produce a test tube chimpanzee-human baby (with either a human or a chimpanzee egg) The result would certainly be interesting, even if the egg could not be fertilized, although there may be difficulties in getting it past an ethics committee, so any attempt would probably have taken place in a country not troubled with such things.

There were attempts to produce a humanzee (by artificial insemination), or some other human-ape product, in the 1920s in the Soviet Union. These are reported as unsuccessful.

I am going to assume that humans and chimpanzees can not breed i.e that they are distinct species.

It would be a fair bet that the hominid and chimp lines diverged at about the same time the hominid chromosome count reduced to 23 pairs. However, different chromosome numbers is not an absolute bar to reproduction.

Horses (32 pairs of chromosomes) and donkeys (31 pairs of chromosomes) can have sex and that can result in the birth of live offspring. The foal of a male donkey and female horse cross is called a mule. Very rarely a female mule is fertile and can have a foal, although not with a male mule as father as male mules are always sterile. The foal of a male horse and female donkey cross is called a hinny. These are never fertile.

So, there is no reliable estimate of when the hominid and chimp lines diverged from interbreeding sub-populations to distinct species.

Looking at things from the other end (our end) it seems that modern humans are of the same species as Neanderthal as Neanderthal DNA is found in “our” DNA (good evidence of interbreeding).

Homo erectus appeared in the fossil record about 1.8 million years ago and disappeared about 300,000 years ago.

Ardi, who lived about 4.4 million years ago in Ethiopa had a small brain (300-350cc) about the size of a modern female chimpanzee. It is estimated that Ardi stood about 1.2 metres tall (4 feet) and weighed about 50kg (110lb)

Lucy who lived about 3.2 million years ago, also in Ethiopia, had a somewhat larger (400-550cc) brain, still a lot smaller than that of the modern human (1200cc ). Lucy is estimated to have been about half the weight of Ardi, about a foot shorter, and to have been fully bipedal.

It is impossible to know whether Ardi and Lucy are direct human ancestors or cousins.

Could a modern human breed with any or all of our ancestors, back to Ardi who lived 4.4 million years ago, say, or even further back? These ancestors have been given a number of names, but are they actually a different species to us?

The period of time for speciation may be a lot less than six million years or 400,000 generations, and there is a lot to accomplish – 1,400 different genes and differences in the base sequences of 20,000 more.

Moreover, all these changes have taken place in a small population, sharing a common environment, and quite possibly interbreeding for prolonged periods.

Lots of change, not much time, small populations.

The wide variety of different physical forms (classified by anthropologists as different species because they look different rather than because of DNA differences) is quite possibly merely evidence of interbreeding

Darwin’s theory of evolution by natural selection says that this rapid evolutionary change is a combination of large numbers of new mutations that have arisen by chance together with the action of natural selection on those mutations (and other random genetic changes).

It is interesting to note that there is very little (no?) fossil evidence of 'chimp evolution' from the common ancestor of six million years ago to the modern chimpanzee of today.

Is it possible that hominids evolved independently in Asia and Africa and the two sub-populations began to interbreed when hominids emigrated from Africa?

The extinction of dinosaurs about 65 million years ago freed up many habitats, including forest floors.

55 million years ago primates were present in Europe, Asia, and Africa. These were small (in the order of 1kg weight) animals adopted to life in trees, with

• fingernails and padded fingertips rather than claws
• five fingers and five toes
• separate radius and ulna in the upper lims and tibia and fibula in the lower limbs.
• forward facing eyes
• opposable thumbs and big toes

Sometime around 45 million years there was a mass extinction of mammalian species because of climate change. Primates almost disappeared from Europe and Asia. However, they then flourished during the Miocene period (from perhaps 25 million years ago to 5 million years ago), when the first hominoids appeared.

Hominoids are the ancestors of the apes, including gibbons, (as opposed to hominids, the ancestors of humans)

Hominoid fossils have been found in Africa, and only Africa, from the early Miocene period, say from 20 million years ago to 13 million years ago. Then there is a gap, with hominoid fossils appearing again seven million years ago.

Hominoid fossils are found in Europe from 13 million years ago, to five million years ago (the middle and late Miocene) and in Asia in the late Miocene period.

One possibility is that a population of hominoids migrated out of Africa, to Asia, (with those tremaining in Africa becoming extinct) and then millions of years later part of the Asian population migrated back to Africa.

There is no question that orang-utan are Asian great apes. The question is what happened in Asia after their ancestors migrated there? The answer must lie in Asian hominoid fossils.

Similarly, gibbons are only found in South East Asia. One of their characteristics is brachiation, or the use of arms (swinging between trees) for locomotion.

The question is, did the population of apes which remained in Asia after the return to Africa of a number of its members, subsequently split into two or more species, one of which became the pongo (orang utang) and the other a hominid line which, in an example of convergent evolution, became the Homo erectus of Asia?

readings

Carroll S.B. Genetics and the making of Homo sapiens. Nature. vol 422. 24 April 2003