The typical narrative about the asteroid-driven mass extinction that occurred 65 million years ago is that it killed off all the dinosaurs and enabled the era of the mammals. That could just be what the dinosaurs want you to think, though. From a different perspective, the aftermath of the extinction saw an explosion of dinosaur diversity, producing the greatest number of living species among any group of tetrapods (vertebrates with four limbs).
We just happen to call those species birds.
The fact that the origins of major groups of birds was so sudden makes it difficult to figure out the evolutionary relationships among them, as the fossil record, when present, shows groups appearing at roughly the same time. A recent attempt to sort out relationships via DNA sequences put some species in close proximity that some in the field found surprising. Now, a new paper is out with a different approach—it rearranges the tree a bit and suggests that many modern birds are descendants of a raptor-like ancestor.
The contrast between the techniques can be considered a difference between depth and breadth. The earlier effort, which we'll call depth, involved obtaining complete genomes from 40 different species of birds. The new one goes broader; it uses only a portion of the genome (a bit under a fifth of a typical bird-sized genome), but analyzes data from nearly 200 species.
The problem breadth is supposed to solve is something called "long branch attraction." It's caused by a basic feature of DNA: when a single base mutates, it only has three possible options that it can change into. As a result, lineages that aren't closely related can, over time, start to look a bit like each other. This is especially problematic when the two branches were separated deep in time (hence, the "long branch" of the name).
The best way to deal with this is simply to add more species. These tend to break up the branches and create clusters of species where there were once individual ones. The clusters more clearly show which groups are most closely related. By analyzing roughly five times as many species, some of the groups of the earlier analysis ended up being broken up and rearranged.
The results are, on some levels, fairly intuitive. All existing water birds—diving, wading, and shore-based—end up in a single group. So do all cranes and the species we'd already thought were their close relatives. When the dates of fossils are used to analyze the time when groups branched off, the results are consistent with a rapid diversification after the mass extinction. In fact, one lineage, which currently contains a single species (the Hoatzin), appears to have branched off from other bird species 64 million years ago.
The results also suggest that the majority of the birds we're familiar with may be the descendants of a raptor-like predator. (We're talking modern raptors, like eagles, rather than the Jurassic Park sort). Hawks are a sister group to land birds. Falcons are a sister group of parrots and their relatives. The fact that all these groups have a raptor as a relative seems to indicate that these groups all had a raptor ancestry.
Obviously, there's still the possibility that looking at more species will shuffle the branches of the evolutionary tree a bit further. And further fossil finds could shift the dates as well. But for now, this seems to be the last word on the origin of the birds, a question the authors call "the greatest unresolved challenge in dinosaur systematics."
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