Great apes have 24 pair chromosomes, humans have 23.

HSA2 fusion 23 paired chrom. Homo, 24 in great apes
BMC Genomics volume 23, Article number: 616 (2022) Cite this article

Abstract
Background
The reduction of the chromosome number from 48 in the Great Apes to 46 in modern humans is thought to result from the end-to-end fusion of two ancestral non-human primate chromosomes forming the human chromosome 2 (HSA2). Genomic signatures of this event
are the presence of inverted telomeric repeats at the HSA2 fusion site and a block of degenerate satellite sequences that mark the remnants of the ancestral centromere. It has been estimated that this fusion arose up to 4.5 million years ago (Mya).

Results
We have developed an enhanced algorithm for the detection and efficient counting of the locally over-represented weak-to-strong (AT to GC) substitutions. By analyzing the enrichment of these substitutions around the fusion site of HSA2 we estimated its
formation time at 0.9 Mya with a 95% confidence interval of 0.4-1.5 Mya. Additionally, based on the statistics derived from our algorithm, we have reconstructed the evolutionary distances among the Great Apes (Hominoidea).

Conclusions
Our results shed light on the HSA2 fusion formation and provide a novel computational alternative for the estimation of the speciation chronology

https://bmcgenomics.biomedcentral.com/articles/10.1186/s12864-022-08828-7

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