Human chromosome 2 was formed by the fusion of two ancestral ape chromosomes.
Evidence for this fusion includes remnants of a second, inactive centromere and telomere-like DNA sequences within the chromosome. Normally, telomeres mark chromosome ends, but chromosome 2 contains an internal telomeric region consistent with an ancient end-to-end fusion.
This event helps explain why humans have 46 chromosomes while great apes such as chimpanzees and gorillas generally have 48. The fusion occurred in the hominin lineage after it diverged from the lineage leading to other living apes, although its exact date cannot be established from the fusion alone.
A common mix-up is saying that chromosome 2 is simply the largest human chromosome. Chromosome 1 is larger; chromosome 2 is notable because its structure preserves unusually strong evidence of ancestral chromosome joining.