R M Cullen
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There are biologists who say that there was no first human, no first homo sapiens. Their argument goes like this: according to standard evolutionary theory the change from one generation to the next is very very small. So small as to be undetectable. Richard Dawkins gives the example of the transition from middle to old age. People do get older, but there is no one night when someone goes to bed middle aged and wakes up elderly. Similarly, using this analogy, there is no one animal whose parents are not human but who is human.
This approach just reflects a sloppiness with definition, or overlapping definitions. The first human is the first great ape with a human defining characteristic (i.e one found in no other great ape) who is a common ancestor to all humans.
We know, with all the certainty of standard evolutionary theory, that a "first human" existed, even if we don't know when he or she lived or even where he or she lived. We can estimate both the when and the where, but neither is certain. The only argument is about which 'human defining' characteristic appeared first.
Humans have 23 pairs of chromosomes. Other great apes have 24. In humans, two chromosomes have fused into one. So the first human can be defined as the first great ape with 23 pairs of chromosomes, the "first fused chromosome ape". This fusion of chromosomes is, according to standard evolutionary theory, an incredibly rare event. It happened in only animal. This animal is the ancestor of all humans, until a better 'human defining characteristic' is found.
This 'first fused chromosome ape' could only have had descendants through breeding with a non-human. Some of its children would have had 23 pairs of chromosomes, some would have had 24. That is, our first human could have both chimpanzee and human descendants alive today. Indeed, he or she belonged to two 'modern' species, human and chimpanzee.
Professor Dawkins makes the mistake of assuming that an animal can belong to only one species. This "first fused chromosome ape" may have been a member of three species - human, chimpanzee, and the common ancestor of humans and chimpanzees. It is a truism in standard evolutionary theory that parents belong to the same species as their children. So, if a chimpanzee alive today has this "first fused chromosome ape" as an ancestor than that animal was a chimpanzee. All humans have this animal as an ancestor so it is human. It is also a truism in evolutionary theory that children belong to the same species as their parents. So this "first fused chromosome ape" also belonged to the same species as its parents. However, its parents were not human as they had 24 pairs of chromosomes.
This "first fused chromosome ape" presents a significant problem for standard evolutionary theory, as he or she would have had reduced fertility. The only available same-species sexual partners were troop members with 24 pairs of chromosomes, and our current understanding of biology says that matings between animals with different chromosome numbers are less likely to result in fertile offspring.
However, once this candidate first human had had children, and either those children, or their children, mated with each other there would have been a population of "early humans", great apes with only 23 pairs of chromosomes. From this time, the difference in chromosome numbers acted to maintain the 'other great ape' and 'early human' populations in a a state of relative reproductive isolation, even if there were mating events between them. The reason is that sex between the two groups was much less likely to result in fertile offspring than sex within the two groups.
This "first fused chromosome ape" presents a significant problem for anthropologists as between it and modern humans lie millions of years of evolution. For example, it is likely that Home erectus had 23 pairs of chromosomes, and would be human as today's humans, tracing their ancestry back, would have erectus ancestors (and as parents are of the same species as their children, erectus must be human). Yet these ancestors are all treated as different species because they had anatomical differences, rather than genetic differences that made a difference.