For many years, scientists have noticed that flying animals, especially birds and bats, are light in the DNA department—they have small genomes. There was always a chicken-and-egg question as to whether smaller genomes (with relatively little DNA in their cellular nuclei) enabled flight or just happened to be a characteristic of animals that flew—whether, in the words of one evolutionary biologist, "they had to jettison their genomic baggage" to fly, "or it was never loaded in the first place."
The answer to that question is in a new Nature paper, which shows that the dinosaur ancestors of modern birds had small genomes well before they turned into anything that could fly. Tyrannosaurus rex, for example, had a bird-sized genome; that is, there was only about one-third as much DNA in each of its trillions of cells as there is in each of ours. The study is an example of how a seemingly random genetic shift can enable a quantum leap in behavior—in this case, flight.
A team led by Chris Organ and other scientists at Harvard performed the Nature study by estimating cell size from measurements of the holes in fossilized dinosaur bones. They compared the cell sizes—which are known to correlate with genome size—to those of living animal relations and used that to group various dinosaur lineages by genome size.
They found that bird-sized genomes were present in the saurischian order of dinosaurs—critters like T. rex, Deinonychus, and the Oviraptors—but not in the confusingly named ornithischian ("bird-hipped") dinosaurs, such as Triceratops, Psittacosaurus, and pachycephalosaurs. These latter dinosaurs have hip rotation similar to that of birds (thereby confusing the 19th-century biologists who named them), but they are the cousins of crocodiles and other reptiles and not directly ancestral to birds.
The smaller genome size probably facilitated higher metabolic activity. That makes sense, since T. rex and colleagues must have been very active. "They wouldn't have been lethargic, lazy things sitting around like the reptiles we see today," says Austin Hughes, a professor of evolutionary biology at the University of South Carolina. The small genome also, coincidentally, permitted the development of flight. The first flying creatures either glided from tree to tree or got a running start and took off. In either case, a small genome would have been an asset: Relatively large genomes slow you down because they require larger nuclei and thus larger cells. Larger cells require more energy to perform cell division. This is one reason that animals with higher metabolism, such as birds and bats, tend to have smaller genomes and cells than their neighbors in the tree of life.