Yes. Strong molecular evidence indicates that human chromosome 2 formed when two chromosomes present separately in a shared primate ancestor fused end to end.
This is assessed at both SL and HL in A3.1 Diversity of organisms, particularly through evaluating evidence for chromosome fusion.
Humans normally have 46 chromosomes, whereas chimpanzees and other great apes normally have 48. This difference can be explained by the fusion of two ancestral chromosomes, referred to in the IB syllabus as chromosomes 12 and 13, to produce one larger chromosome in the human lineage. The equivalent DNA remains on two separate chromosomes in chimpanzees, often called chromosomes 2A and 2B.
Evidence | How it supports fusion |
|---|---|
Matching gene sequences | Genes on human chromosome 2 occur in approximately the same order as genes distributed across the two corresponding chimpanzee chromosomes. This conserved arrangement is called synteny. |
Internal telomere sequences | Telomeres normally occur at chromosome ends, but human chromosome 2 contains telomere-like repeats near its middle-the predicted position of an end-to-end fusion. |
Two centromere regions | A normal chromosome has one functional centromere. Human chromosome 2 has one active centromere and remnants of a second ancestral centromere. |
Chromosome number | Fusion reduced the diploid chromosome number from 48 to 46 without requiring the loss of a large quantity of genetic information. |
A common misconception is that two chromosomes from modern apes fused to make a human chromosome. Humans are themselves apes, and the fusion involved ancestral chromosomes after the human lineage diverged from the lineage leading to chimpanzees, notnot chromosomes taken from present-day chimpanzees.
Exam technique: If asked to “evaluate the evidence,” identify several independent observations and explain how each matches a prediction of the fusion hypothesis. Do not rely only on the difference between 46 and 48 chromosomes; internal telomere repeats, the inactive centromere and matching gene order provide stronger molecular evidence.