Sixty-six million years in the past, an asteroid slammed into the Gulf of Mexico. The disaster led to the extinction of as many as three-quarters of all species on Earth, together with dinosaurs like Tyrannosaurus rex. However some flying feathered dinosaurs survived, and finally advanced into the greater than 10,000 species of birds residing as we speak, together with hummingbirds, condors, parrots and owls.
Based mostly on the fossil report, paleontologists have lengthy argued that the asteroid’s impression was adopted by an enormous pulse of chook evolution. The mass extinction of different animals might have eradicated loads of competitors for the birds, giving them the prospect to evolve into the outstanding range of species that fly round us as we speak.
However a new examine on the DNA of 124 chook species challenges that concept. A world group of scientists discovered that birds started diversifying tens of hundreds of thousands of years earlier than the fateful collision, suggesting that the asteroid had no main impact on chook evolution.
“I think about it will ruffle a couple of feathers,” mentioned Scott Edwards, an evolutionary biologist at Harvard and one of many examine’s authors. The analysis was printed on Monday within the Proceedings of the Nationwide Academy of Sciences.
Dinosaurs advanced primitive feathers not less than 200 million years in the past, not for flight however almost definitely for insulation or a mating show. In a single lineage of small bipedal dinosaurs, these feathers grew to become extra complicated after which finally took the creatures into the air as birds. How feathers became wings for flight remains to be debated. However as soon as birds advanced, they diversified into quite a lot of kinds, a lot of which grew to become extinct when the asteroid plunged the Earth right into a yearslong winter.
When trying to find fossils of the key teams of birds alive as we speak, scientists have discovered nearly none that shaped earlier than the asteroid hit. That putting absence has led to a principle that the mass extinctions cleared the evolutionary area for birds, permitting them to blow up into many new kinds.
However the brand new examine got here to a really totally different conclusion.
“We discovered that this disaster didn’t have impression on trendy birds,” mentioned Shaoyuan Wu, an evolutionary biologist at Jiangsu Regular College in Xuzhou, China.
Dr. Wu and his colleagues used the birds’ DNA to reconstruct a household tree that confirmed how the key teams had been associated. The oldest break up created two lineages, one that features as we speak’s ostriches and emus, and the opposite with the remainder of all residing birds.
The scientists then estimated when the branches break up into new lineages by evaluating the mutations that accrued alongside the branches. The older the break up between two branches, the extra mutations every lineage constructed up.
The group included paleontologists who helped to fine-tune the genetic estimates by analyzing the age of 19 fossils of birds. If a department gave the impression to be newer than a fossil that belonged to it, they adjusted the pc mannequin that estimated the tempo of chook evolution.
Michael Pittman, a paleontologist on the Chinese language College of Hong Kong who was not concerned within the new examine, mentioned that it was significantly noteworthy due to the fossil evaluation. “They’d a dream group of paleontologists,” he mentioned.
The examine discovered that residing birds shared a standard ancestor that lived 130 million years in the past. New branches of its household tree steadily break up off all through the Cretaceous Interval and afterward at a reasonably regular tempo, each earlier than and after the asteroid impression. Dr. Wu mentioned that this regular pattern may need been fueled by the rising range of flowering crops and bugs throughout the identical interval.
Jacob Berv, an evolutionary biologist on the College of Michigan who was not concerned within the examine, mentioned that it illustrated state-of-the-art strategies for crunching big quantities of genetic information to reconstruct evolutionary historical past. However he didn’t agree with its conclusion.
If the brand new examine was proper, there ought to be fossils of all main teams of residing birds from properly earlier than the asteroid impression. However nearly none have been discovered.
“The sign from the fossil report isn’t ambiguous,” Dr. Berv mentioned.
Dr. Berv suspects that the right story comes from the fossils, and that almost all main teams of birds emerged after the asteroid impression. The potential downside with the brand new examine, he mentioned, is that it assumes that the chook DNA accrued mutations at a gentle charge from one era to the subsequent.
However the devastation of the asteroid’s impression — inflicting forests to break down and creating shortages of prey — may need led to the deaths of larger birds, whereas smaller birds survived. Small birds take much less time to breed, and they’d produce many extra generations — and plenty of extra mutations — than birds did earlier than the impression. If scientists ignore this sort of mutational overdrive, they are going to get the timing of evolution fallacious.
Nonetheless, Dr. Berv acknowledged that scientists are simply beginning to develop strategies to that might permit them to raised estimate the speed of evolution and combine it with different proof like DNA and fossils. “I think that may reconcile a few of the debates,” he mentioned.
