I have a question regarding Chromosome 2. I've heard that there is evidence that the Human Chromosome 2 is a fusion between the two ancestral chromosomes 2a and 2b, but could it be possible that it is the other way around? That is, could it be possible that what we call chromosome 2 in the last common ancestor between humans and other great apes could have broken in some offspring of this ancestor and preserved the broken copy in their descendants. Meanwhile other descendants of this ancestor kept an intact chromosome? Can someone please point me to evidence/papers that ruled out this possibility? I'd like to hear why they thought it was a chromosome fusion and not a chromosome break, especially with an incomplete archeological record available.

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    $\begingroup$ I've heard that there is evidence that the Human Chromosome 2 is a fusion between the two chimp chromosomes 2a and 2b -- a small but important correction: human's didn't evolve from chimps, so human chromosomes cannot be derived from chimp chromosomes. It is more correct to say that chromosome 2 is hypothesized to have formed by a fusion of two ancestral chromosomes after the divergence of the "human" lineage from the most recent common ancestor of humans and other primates. $\endgroup$
    – acvill
    Jul 7, 2022 at 18:40
  • $\begingroup$ Related: Evolutionarily speaking, why do humans have 46 chromosomes. There is very good evidence that human chromosome 2 is the result of a telomere-to-telomere fusion of two ancestral chromosomes (ref is here) $\endgroup$
    – user338907
    Jul 7, 2022 at 21:33

1 Answer 1


The way you answer these sorts of questions is by looking at other, further relatives because that gives you more hints about what the "ancestral" trait is.

Since other apes (gorillas, orangutans) also have the separate "2a" and "2b" chromosomes (see, e.g., Fan et al 2002), we can conclude that the common ancestor to all of the great apes had separate 2a/2b chromosomes. That makes humans the outlier, and therefore suggests the fusion occurred somewhere between the most recent common ancestor of chimps and humans and modern humans.

The alternative would have to be that the ancestor had a single "chromosome 2", and then separately in every other great ape lineage that chromosome broke in the same place. That is effectively impossible to have occurred in the same way like that by chance.

I've annotated the cladogram from the Wikipedia page on great apes to show how this works:

Annotated cladogram

If the great ape ancestor has separate 2a/2b, then one fusion event is required to explain why humans differ from the others (top diagram).

If, however, the great ape ancestor has a single chromosome 2, which then broke to make 2a/2b in chimps, that's a perfectly reasonable hypothesis if you just look at the chimpanzee/human branch. However, when you also consider gorillas and orangutans that have separate 2a/2b, that would mean that the ancestral chromosome 2 would have had to break at least 3 separate times in 3 separate lineages in the same place. That doesn't make much sense.

Fan, Y., Linardopoulou, E., Friedman, C., Williams, E., & Trask, B. J. (2002). Genomic structure and evolution of the ancestral chromosome fusion site in 2q13–2q14. 1 and paralogous regions on other human chromosomes. Genome research, 12(11), 1651-1662.

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    $\begingroup$ there is also structural evidence in the chromosome itself, chromosome 2 has telomere like sequence in its center and a second inactive centromere. those only make sense if the it was to different chromosomes in the past. en.wikipedia.org/wiki/Chromosome_2 $\endgroup$
    – John
    Jul 7, 2022 at 23:51
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    $\begingroup$ It's important to emphasize, I think, that the unfavorability of the "break" hypothesis arises not merely from the pattern of occurrence in contemporary apes and humans alone, but from that pattern together with the fine details of their past evolutionary relationships. If it were the case that all non-human apes shared their own common ancestor after the one for both them and human apes, only one break would be required, even were the distribution at the leaves identical. But because Hominina splits off where it does, instead of before the first fork in the diagram, 3 breaks are required. $\endgroup$ Jul 8, 2022 at 22:58

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