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.
It’s genome size, not body size, that matters. Most salamanders are smaller than birds, but their genomes are often 90 times larger. If you’ve ever seen a salamander move (they don’t get around much), you can see why they don’t need small cells. Having larger cells also means that salamanders have fewer neurons per square centimeter of brainpan. Thus “bird-brained” is unfair to birds (though “amphibian-brained” doesn’t have the same ring, I grant you).
There are other advantages to smaller genomes and cells. Smaller red blood cells give an organism a higher surface-to-volume ratio for the exchange of gases like oxygen and carbon dioxide. Birds need a very high metabolism to fly.So, it’s not surprising, for example, that the ostrich has a larger genome than the finch.
So, why should anyone besides evolutionary biologists care about all of this? Because it reveals the combination of random and selective changes that make up the history of life. There’s nothing like one of these studies to show that if God is in charge of this show, he’s very vague in his stage directions.
Also, helping us parse evolution is one way in which molecular biology is continually reshaping our ideas about life’s building blocks. When scientists first learned about the smallness of animal genomes several decades back, they were puzzled that small, insignificant creatures like salamanders had much more DNA per cell than people. This phenomenon was described as the “C-value paradox,” with the C-value signifying the size of the genome. At the time, scientists figured that DNA and genes were synonymous—in other words, that all of our DNA had the purpose of encoding proteins. Hence the paradox: Humans were obviously more complex than onions, yet the onion has five times more DNA in each cell than a human being does.
Gradually, scientists realized that most DNA did not code proteins (the current estimate, in human DNA, is about 1.5 percent). People, apes, birds, and corn all have similar numbers of genes. But they have varying amounts of what many scientists, having no better explanation for the stuff, used to call “junk DNA.” Why some of God’s creatures have more DNA than others is pretty much anyone’s guess, but a great deal of research is now focused on learning what the various types of “junk DNA” do or do not do. The junkyard includes things like jumping genes, which can replicate on their own and float around the chromosome, and pseudogenes, relics of old genes that went through so many mutations, they lost their coding mojo. “In many ways, these various elements can be conceived of as living in the ecosystem of the genome,” says T. Ryan Gregory, a professor of integrative biology at the University of Guelph in Ontario. Like an ecosystem, the genome is a jungle of random and selective evolution. Sometimes bits of dead genes come back to life, in a sense, by getting themselves inserted into a spot on a chromosome in a way that disrupts the function of newer genes or affects their expression in the cell.
Other times, the genes just hang in there for awed scientists to gaze upon, like the ruined pleasure palace of Kublai Khan. Though wondrous, to be sure, the DNA ecosystem looks more and more undesigned the more we know about it. The miracle of flight is one example that makes you wonder—what makes a miracle?