When did human chromosome 2 fuse?(2023)

Summary

Fluorescence hybridization image of human chromosomes in metaphase. The probe glowing red is one that binds to telomeric DNA. The chromosome in the zoomed inset is human chromosome 2, which has telomeric DNA near its center, the point where two ancestral chromosomes fused together. Image from Ijdo and coworkers (1991). Most living people have 23 pairs of chromosomes, as did our relatives known from ancient DNA, the Neandertals and Denisovans. All of our closest primate relatives—chimpanzees, bonobos, gorillas, and orangutans—have 24 pairs. From comparing these genomes with each other, it is clear that sometime during human evolution two ancestral chromosomes fused together, reducing our number. The product of that fusion is human chromosome 2, the second-largest of our chromosomes.Evolutionary geneticists often focus closely on differences in chromosome numbers, which can be related to reproductive incompatibility between sister species. Structural changes to chromosomes may also affect gene regulation or other functions. Did the chromosome 2 fusion make a difference to our ancestors, maybe even cause an ancient speciation in our lineage?Maybe the biggest barrier to understanding how this chromosome fusion mattered is a better knowledge of exactly when it happened. Recently geneticists led by Barbara Poszewiecka applied a creative approach to estimate how long ago the two ancestral chromosomes fused together. They found a surprisingly recent range of times: between 400,000 and 1.5 million years ago.A new paper just published suggests that a major bottleneck in our evolutionary history happened between 930,000 and 800,000 years ago, and points to the chromosome 2 fusion as one possible consequence. This interval is a very interesting time. Our African ancestors, Neandertal ancestors, and Denisovan ancestors all diverged from each other around 700,000 years ago—and all these branches share the fused chromosome. It seems likely that the population that gave rise to the...