From Robert Jackson
Catherine Brahic’s discussion of fresh efforts to pin down when humans split from chimps (24 November, p 34) is a nice example of the contribution of quantitative genetics to understanding evolution, but there is another strand to the story of the emergence of humans – the significant qualitative difference between humans and chimpanzees. Humans have 23 pairs of chromosomes rather than the 24 of the great apes.
When this chromosomal rearrangement occurred in a germ cell of our common ancestor, the mother and daughter cells contained identical complements of genes, but the difference in how those genes were packaged would have resulted in changes in how and where genes were expressed.
I have carried out calculations, which I hope to publish in 2013, showing that such chromosomal rearrangements can result in partial reproductive isolation that over many generations may lead to formation of separate species even in the absence of geographic isolation, a process known as sympatric speciation. Proving this hypothesis of human evolution will require karyotyping our protohuman ancestors to show their chromosomal make-up, and for that we need a body preserved in ice. Given our tropical origins, this may not be easy.
• Sympatric speciation remains contentious.
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