|Photo: UC Museum of Paleontology|
Phylogeneticists (in their earlier form called cladists) would measure and score traits like producing milk, having fur, and producing body heat, and show quantitatively that there is a group (mammals) that all come from a common ancestor that had these traits. This is based on the assumption that the simplest explanation for the distribution of these traits is the one that involves the fewest changes across the tree, and that the simplest explanation is the best. So the simpler explanation for the distribution of the traits of milk, fur and body heat across animals is that the ancestor of all mammals had all these traits and passed them on to its descendants, instead of these traits evolving independently many times. Phylogeneticists had to deal with difficulties like the fact that birds also produce body heat (convergence), but by carefully scoring information about traits, they could tell us that whales are not fish and bats aren't bugs.
|Things got substantially more intense|