Today, there are two-toed and three-toed sloths, very similar animals living in very similar environments. So most people would assume they are each others' nearest relatives. But they're only nearest surviving relatives, and the extinct family members constitute most of the history -- many dozens of extinct ground sloths ! And three-toed sloths diverged from the rest of the taxon not long after anteaters did, while two-toed sloths seem to be at the end of a long chain. That's why taxonomy based only on living relatives (as originally done by Linnaeus) led to so many, um, "misunderstandings". Linnaeus (or maybe it was Cuvier) originally lumped elephants, rhinos, and hippos together in the "Pachydermata". Now they're known to be less closely related to each other than to hyraxes, horses, and whales, respectively ! For those of us who don't specialize in such things, much of this is rather surprising, even disconcerting, news.
About twenty years ago I attended a 'general-interest' (i.e. not just for specialists, so even though a chemist, I didn't get lost in "insider" jargon) academic seminar on modern DNA analysis among living birds, and how it completely revised much of the taxonomy of birds. Large flightless birds (ostriches, emus, cassowaries, rheas) turned out not to be as closely related as thought -- apparently flightlessness evolved repeatedly among different, related lineages, so that flightless and flying birds were mixed within various taxons. Old World and New World vultures -- whose relationships were considered problematic, unknown to those of use outside ornithology -- turned out to have different lineages, another study in convergent evolution. And there was a suggestion that NW vultures descended from cranes, but this now seems to have been retracted or discarded. (Too bad I can't follow such developments in detail and still pursue my own profession.) In any case, I was much impressed with the way modern, detailed DNA analysis and cladistics (which was still something of a "new thing" when I first heard of the practice) had untangled a lot of confusion. Previously, I had considered much of the constant revision of taxonomies to be driven often by opinions as much as evidence. But here was reproducible, quantifiable evidence put to good use, and not much to argue in opposition. Quite a sea change. It makes one sorry one can't pursue multiple careers in different branches of science, just to experience all the progress that is otherwise so impressive to the specialist, and unknown "outside".
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