Modern birds originated around 95 million years ago in South America, according to researchers who created a "time tree" using both fossils and genomic data from hundreds of birds. Their findings, published in Science Advances this week, show that plate tectonics and global climate change left an imprint on the evolution of modern birds.
When it comes to species richness and distribution, our feathered friends are the most diverse group of terrestrial vertebrates, yet many aspects of their evolutionary history are still being hotly debated. For example, while most researchers agree that modern birds originated some time in the Cretaceous period, their estimates vary from 72 million to 170 million years ago.
To better understand how birds arose, Santiago Claramunt and Joel Cracraft from the American Museum of Natural History combined data on 130 fossil birds with DNA sequences from 230 species spanning all avian orders and hundreds of avian families. This allowed them to generate a time tree – an evolutionary tree with temporal information – that illustrates the branching (or diversification) of modern birds. Fossils are used to put dates on some parts of the tree, and DNA sequences combined with statistical methods are used to extrapolate the timing information to the rest of the tree.
They discovered that the most recent common ancestor of all of today’s birds lived in what is now South America on West Gondwana around 95 million years ago in the early Late Cretaceous. But it wasn’t until roughly 66 million years ago during the Cretaceous-Paleogene (K-Pg) transition that the ancestors of modern birds began to diversify rapidly around the world, the duo’s biogeographic models show. The K-Pg extinction was when all non-avian dinosaurs were wiped out.
Using land bridges, the birds expanded to various continents through two main dispersion routes: They reached the Old World (Africa, Europe, and Asia) through North America, and they made their way to Australia and what is now New Zealand through Antarctica. Their diversification was heavily influenced by continental shifts and the changing configuration of landmasses, from seafloor spreading to the uplift of mountain ranges.
When the team looked for patterns linking bird evolution with global climate, they found that diversification rates increased during periods of cooling and decreased during periods of warming. Cooler temperatures likely caused the widespread retraction and fragmentation of tropical forests. Isolating populations puts them at risk of extinction, but it also sets the stage for the evolution of new species.
For speciation rates to be higher than extinction rates, the fragmentation should be moderate. "Tropical biomes are fragmented but those fragments should be large slabs of habitat, in which populations can persist," Claramunt explains to IFLScience. But if the fragments are too small, "extinction rates may surpass speciation rates, and the pattern would be the opposite." Warming, on the other hand, increases connectivity. "Speciation requires a period of isolation; if populations remain connected, speciation cannot happen," he adds. "At the same time, extinction may also be low because populations are large and interconnected, preventing local extinction." So while warming may help species survive, it doesn’t stimulate speciation.