Wednesday, December 16, 2015

The Dawn of Birds

Fig.1. A green-naped lorikeet, a little owl, an Adélie penguin, and a northern cardinal show only a fraction of the remarkable diversity of birds (Credit: Benjamint, 2009Trebol-a, 2011Reinhard Jahn, 2007; & Stephen Wolfe, 2011)

Around 65 million years ago, one of the largest mass extinction events in Earth’s history occurred. An estimated 75 percent of all species went extinct, from non-avian dinosaurs to types of mollusks, plankton, insects, and plants. With extinction, however, came the chance for animals like birds to diversify and expand into empty ecological niches in a process called adaptive radiation. Although the Cretaceous-Paleogene extinction event (K-Pg) may have enabled the rapid evolution of new species of birds, research published last Friday in the journal Science Advances suggests that birds have a much longer and more complicated history than previously thought.

Lead author Santiago Claramunt and his colleague Joel Cracraft, both researchers at the American Museum of Natural History in New York, generated a new evolutionary tree for these dinosaur relatives through mapping DNA sequences with known climate data from millions of years ago. They completed the first quantitative analysis that estimated where birds might have arisen with a comparison of 130 fossil birds and modern bird DNA sequences.

Fig.2. The evolution of Darwin’s famous finches was strongly impacted by their geography and is a common example of adaptive radiation (Credit: John Gould, 1845)

Although some researchers suggest that modern birds did not diversify until after the K-Pg extinction event, Cracraft pointed to large gaps in the fossil record. The majority of modern bird fossils date to after 65 million years ago but previous genetic analyses point to a longer evolutionary history for these animals.

Claramunt's and Cracraft's DNA analysis revealed that birds arose and diversified in modern-day South America around 95 million years ago and radiated extensively around the time of the K-Pg extinction. This latter diversification supports the previous thought that a mass extinction could spur on rapid evolution, but additional and more nuanced story in the climate data had more to say.

When the Earth cooled and dried, fragmentation of tropical forests probably isolated bird populations. Smaller populations would have gone extinct, but those that survived would have expanded and propagated when environments warmed. Previously studied sediment cores revealed several periods of cooling between 65 and 10 million years ago that correlated with rapid diversification described by Claramunt and Cracraft.

Fig.3. The continents of Earth looked much different 65 million years ago than today (Credit: Ron Blakey, Colorado Plateau Geosystems, Arizona USA)

In addition to climate data, Claramunt and Cracraft studied tectonic models that showed how birds might have left South America via land bridges and spread around the world. Although some birds could certainly fly regardless of land bridges, land provided food and shelter, especially for those birds who could only travel short distances or did not feed on marine life. Birds probably expanded into North America and then the Old World via the Bering land bridge. Claramunt and Cracraft think that birds then traveled to Australia and New Zealand across Antarctica, which was relatively warm at that time.

Recent studies by natural history museum researchers have sought to understand the evolutionary underpinnings of birds. This research delves into the geography and climate behind bird movement and evolution in an effort to combine the fossil record and genetic analyses. Environmental records, genetic samples, and fossil birds were all crucial to creating a more detailed evolutionary history for these animals.

Claramunt, S., & Cracraft, J. (2015, Dec 11). A new time tree reveals Earth’s history imprint on the evolution of modern birds. Science Advances, vol. 1 (11): e1501005. doi: 10.1126/sciadv.1501005


adaptive radiation
A process by which organisms evolve and diversify rapidly, often resulting in new species.
Cretaceous-Paleogene (K-Pg) extinction event
Also called the Cretaceous-Tertiary extinction (K-T), this event was the mass extinction of most of the plant and animal species on Earth around 65 million years ago.
Bering land bridge
An ancient land bridge that connected Asia and North America at various times during the last Ice Age.

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