Sixty-six million years in the past, an asteroid crashed 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 resembling Tyrannosaurus rex. However some flying feathered dinosaurs survived, and finally developed into the greater than 10,000 species of birds alive as we speak, together with hummingbirds, condors, parrots and owls.
Based mostly on the fossil document, paleontologists have lengthy argued that the influence of the asteroid was adopted by an excellent impulse within the evolution of birds. The mass extinction of different animals could have eradicated a lot of the competitors for birds, giving them the chance to evolve into the exceptional variety of species that fly round us as we speak.
However a brand new examine on the DNA of 124 species of birds challenges this concept. A world group of scientists discovered that birds started to diversify tens of hundreds of thousands of years earlier than the deadly collision, suggesting that the asteroid didn’t have a significant impact on the evolution of birds.
“I think about this may ruffle a couple of feathers,” mentioned Scott Edwards, an evolutionary biologist at Harvard and one of many examine's authors. The analysis was printed Monday within the Proceedings of the Nationwide Academy of Sciences.
Dinosaurs developed primitive feathers at the very least 200 million years in the past, not for flight, however almost definitely for insulation or a mating show. In a lineage of small bipedal dinosaurs, these feathers grew to become extra complicated after which took the creatures into the air as birds. How feathers grew to become wings for flight remains to be debated. However as soon as birds developed, they diversified into a wide range of types, lots of which grew to become extinct when the asteroid blasted Earth into an extended winter.
When on the lookout for fossils of the big teams of birds alive as we speak, scientists discovered virtually none that shaped earlier than the asteroid. This placing absence has led to a principle that mass extinctions cleared the evolutionary enviornment for birds, permitting them to blow up into many new types.
However the brand new examine got here to a really completely different conclusion.
“We discovered that this disaster had no influence 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 earliest cut up created two lineages, one that features as we speak's oysters and emus, and the opposite with the remainder of all dwelling birds.
Scientists have estimated when the branches cut up into new lineages by evaluating the mutations which have amassed alongside the branches. The older the cut up between two branches, the extra mutations every lineage has constructed.
The group included paleontologists who helped refine the genetic estimates by inspecting the ages of 19 chicken fossils. If a department seemed to be newer than a fossil it belonged to, they adjusted the pc mannequin that estimated the speed of chicken evolution.
Michael Pittman, a paleontologist on the Chinese language College of Hong Kong who was not concerned within the new examine, mentioned it was notably noteworthy due to the fossil evaluation. “That they had a dream group of paleontologists,” he mentioned.
The examine discovered that dwelling birds shared a standard ancestor that lived 130 million years in the past. New branches of its household tree cut up off steadily all through the Cretaceous after which at a reasonably common charge, each earlier than and after the asteroid influence. Dr Wu mentioned this regular pattern could possibly be fueled by the rising variety of flower crops and bugs over the identical interval.
Jacob Berv, an evolutionary biologist on the College of Michigan who was not concerned within the examine, mentioned it illustrated cutting-edge strategies for crunching enormous quantities of genetic information to reconstruct evolutionary historical past. However he doesn’t agree together with his conclusion.
If the brand new examine was proper, there must be fossils of all the key teams of birds dwelling from lengthy earlier than the influence of the asteroid. However virtually none had been discovered.
“The sign from the fossil document isn’t ambiguous,” mentioned Dr. Berv.
Dr. Berv suspects that the proper historical past comes from the fossils, and that a lot of the main teams of birds emerged after the influence of the asteroid. The potential downside with the brand new examine, he mentioned, is that it assumes that the birds' DNA has amassed mutations at a gentle charge from one era to the subsequent.
However the devastation of the asteroid influence – which prompted the collapse of forests and created a scarcity of prey – may have led to the dying of bigger birds, whereas smaller birds survived. Small birds take much less time to breed, and produce many extra generations—and plenty of extra mutations—than birds did earlier than the influence. If scientists ignore this sort of mutational overdrive, they may get the timing of evolution unsuitable.
Nevertheless, Dr. Berv acknowledges that scientists have simply begun to develop strategies that would enable us to raised estimate the speed of evolution and combine it with different proof resembling DNA and fossils. “I think this may settle a number of the debates,” he mentioned.
