Humans have 46 chromosomes, while chimpanzees have 48. At first glance, the difference might seem like a loss of genetic material. However, the clue that explains this count is in our chromosome 2: much of its content corresponds to that of two separate chromosomes in other great apes.
One of the most striking signals appears in an interior region of the chromosome. There are sequences similar to those of telomeres, which are normally at the ends of chromosomes, arranged in facing orientations. It is the type of trace that would be expected if two ancestral chromosomes had been joined at their ends. Comparing the relative position of DNA blocks with that of other primates helps piece together that story.
A useful analogy is that of two notebooks being rebound into one. Even if some pages are moved or modified, blocks can still be recognized and compared to those in other notebooks. Likewise, the organization of DNA preserves clues about changes that have occurred throughout evolution.
This fusion does not alone explain what made us human nor was it the sole cause of the differences with chimpanzees. It is, rather, a trace of our shared history. Also remember an important idea: having fewer chromosomes does not automatically mean having less genetic information.
Santiago Campillo (@Scruzcampillo) is a biologist and scientific communicator specialized in health, nutrition and nature.
