Human chromosome 2 is the fused chromosome formed when two ancestral ape chromosomes joined end to end.
Humans have 23 pairs of chromosomes, while the other great apes generally have 24 pairs. Comparisons of chromosome sequences show that human chromosome 2 corresponds to two separate chromosomes found in chimpanzees, gorillas, and orangutans. This fusion occurred in the lineage leading to modern humans after it diverged from the lineage leading to other living apes.
A key clue is the presence of remnants of telomeres, the chromosome-end sequences, near the middle of human chromosome 2. Researchers also identified an inactive second centromere, consistent with a fusion between two ancestral chromosomes.
Chromosome fusion is a chromosomal rearrangement, not a complete explanation for human evolution by itself. It is one line of evidence for common ancestry, alongside DNA sequence similarities, shared genetic errors, and comparative anatomy. The fusion is estimated to have happened millions of years ago, before the emergence of modern Homo sapiens.