JOIN US'Big bang' in bird evolution revealed
A massive genetics project has produced the most comprehensive bird family tree ever, an embarrassment of scientific riches for studying everything from how birds evolved so quickly after dinosaurs disappeared to the ways in which birds and people learn.
The biggest takeaway from the eight studies published in the journal Science is the way genetic codes can be used to answer wide-ranging questions. Scientists are using birds’ DNA, for instance, both for research on the brain and learning and to reconstruct what an ancient ancestor of birds and dinosaurs might have looked like.
Researchers mapped the complete set of DNA instructions, or genomes, of 45 bird species representing every group of living birds, as well as representatives of all three groups of crocodilians – the closest living bird relatives. To produce the new family tree, scientists combined this information with previously published sequences for zebra finches and domestic turkeys and chickens.
Most of our notions of how DNA evolves over time come from studying the genetic instruction books of mammals, says Ed Green, a genome scientist at the University of California, Santa Cruz.
Adding the genomes of birds and crocodilians – the American alligator, saltwater crocodile and Indian gharial – now allows scientists to better understand how these groups are related, he says.
The new study finds the rate of change in birds’ DNA took off 66 million years ago when most dinosaurs went extinct. The surviving dinosaurs then radiated into a constellation of species that led to about 95 percent of birds on the planet today, says Erich Jarvis, a neurobiologist at Duke University in Durham, North Carolina.
An odd duck
That accelerated rate of change in birds – as compared with the rate in “living fossils” like crocodilians and the common ancestor of birds and crocodilians – makes them the odd ducks on this branch of the family tree, says David Ray, a biologist at Texas Tech University in Lubbock.
And while birds evolved at similar rates as mammals, bird genomes are only about one-third the size, he says. They have all the same basic functions as mammals, though, including the capacity for vocal learning. In fact, one of the exciting parts of this massive project is what Jarvis and colleagues have learned about how birds and people learn unique vocalisations.
Researchers have also pinpointed areas of the brain involved similarly in vocal learning in both birds and people. Similarities in vocal-learning genes in both groups will allow scientists to use birds in studies of conditions such as stuttering or Parkinson’s in people, Erich says.
Now that his team has identified the genes for vocal learning in birds and
people, Erich wants to see if he can start manipulating the genes with an eye to
repairing damaged ones.
Ancient biology
Another revelation is the relatively rapid rate at which birds evolved compared with their crocodilian relatives, which have remained essentially unchanged for over 100 million years.
By comparing the genomes of birds with those of reptiles, researchers found that the common ancestor to birds, crocodilians, and dinosaurs also evolved relatively slowly. This ancestor, in a group called the archosaurs, lived roughly 240 million years ago.
Getting a handle on evolutionary rates gives paleontologists an idea of archosaurs’ generation times, or how soon the animals could reproduce. Slower evolutionary rates generally go hand-in-hand with longer generation times, says the University of California’s Ed. That’s the kind of life history information that doesn’t fossilise, he says.
Researchers were also able to reconstruct about half of the archosaur genome based on what they found for birds and crocodilians. And that’s one of the most exciting things about these studies, says Ed. “We will never get DNA directly from these organisms,” he says. “It’s gone, it’s lost to history.” But if scientists can reconstruct the archosaur’s genetic code, they can start to study questions such as how these animals reproduced.
Sex isn’t determined genetically for crocodilians in the same way that X and Y chromosomes dictate whether a human embryo becomes a boy or girl, Green says. Instead, environmental temperatures during a crocodilian embryo’s time in its egg determine whether the young animal becomes male or female.
Did archosaurs operate in a similar fashion to modern-day crocodiles? It’s a question that could be answered once researchers can get a better handle on how sex determination works in crocodilians and compare that with a completed archosaur genome.