What is the longest human chromosome called?

Chromosome 1, and the naming is exactly that literal - they were numbered by size when they could first be distinguished under a microscope, largest to smallest. Chromosome 1 carries something like eight per cent of the entire genome, around 2,000 genes, and mutations in it are implicated in several hundred diseases.

The numbering has one embarrassment in it. Chromosome 21 is actually smaller than 22, and the mistake was noticed after Down syndrome had already been established as trisomy 21 - so the numbers were left alone rather than renaming a condition in every medical textbook in the world.

The other interesting one is chromosome 2. Humans have 23 pairs and all the other great apes have 24, and the difference is that human chromosome 2 is two ape chromosomes fused end to end - which is visible in the DNA, because it contains the remains of telomeres, the caps that normally sit only at the ends, stranded in the middle, plus traces of a second centromere. It is about as direct a piece of evidence for common ancestry as biology offers.