Sometime in our evolution, two separate chromosomes fused into one, giving us a karyotype of 46 chromosomes where chimpanzees, bonobos and gorillas have 48 chromosomes. The high-coverage genome was sufficient to show that Denisova shared the human fusion:
Of more relevance may be examination of aspects of the Denisovan karyotype. The great apes have 24 pairs of chromosomes while humans have 23. This difference is caused by a fusion of two acrocentric chromosomes that formed the metacentric human chromosome 2 (25), and resulted in the unique head-to-head joining of the telomeric hexameric repeat GGGGTT. A difference in karyotype would likely have reduced the fertility of any offspring of Denisovans and modern humans. We searched all DNA fragments sequenced from the Denisovan individual and identified twelve fragments containing joined repeats. By contrast, reads from several chimpanzees and bonobos failed to yield any such fragments (8). We conclude that Denisovans and modern humans (and presumably Neandertals) shared a karyotype consisting of 46 chromosomes.
We still have no idea whether this fusion made any difference to any phenotype in ancient humans.
Many, many people have written me over the years to ask whether this fusion of two ancestral chromosomes might have been important to our evolution. Perhaps, many suggested, if Neandertals had a chromosomal incompatibility with us, that would explain why they became extinct. I have always doubted this, but without information it was impossible to be certain.
It's nice to now have the information in hand: This fusion happened earlier in our evolution.
- A High-Coverage Genome Sequence from an Archaic Denisovan Individual. Science. 2012 ;338(6104):222-226.