February 12, 2013

For Darwin’s Birthday, Scientists Reconstruct an Important Evolutionary Ancestor

by Julia Rosen

Charles Darwin’s early sketch of the tree of life. Credit: Wikimedia Commons

Two hundred and four years ago today, Charles Darwin was born in Shrewsbury, England. He grew up, of course, to become the father of evolution –— the first to propose a robust, testable scientific hypothesis to explain the origin and diversification of species. After more than a century of research on the subject, the principle of evolution stands as one of the most fundamental facts of life on Earth: all organisms are related through common ancestors in a web of connections known as the tree of life. It is fitting, then, that a major development in our understanding of our place in this tree occurred just last week when a group of 22 scientists led by Maureen O’Leary of Stony Brook University published a paper in the journal Science describing the humble creature from which all placental mammals eventually evolved, including us.

Put simply, a furry insect-eating animal was probably the first mammal to give birth to its young after a period of gestation in which the fetus was nourished in utero by a placenta. From these small beginnings, all living placental mammals evolved, including whales, elephants, and humans. Incredibly, although the authors do not know the identity of this common ancestor — —who may or may not be preserved in the notoriously spotty fossil record — they were able to reconstruct astonishing details of its appearance down to the number and type of teeth it used to feast on bugs. The acclaimed natural history illustrator Carl Buell then transformed this long list of attributes into a meticulous rendering of this ancient creature. “The teeth were indeed drawn individually, the shape and size of the ear, [and the] texture of the fur—including the rough patch over the hips, the length of the tail and the length of the hair on the tail!” he explained in an email.

The reconstructed appearance of the oldest common ancestor was one of the major results of the study. Credit: Carl Buell

This reconstruction was possible because the scientists identified this ancestor through a morphological analysis in which they analyzed the anatomical characteristics of living and fossil placental species to determine the shared traits their common ancestor must have possessed. Although the technique itself is not new, this study is remarkable for its complexity and scale: the authors investigated over 4,500 characteristics in 86 species with the help of MorphoBank, a web-based interface that allowed for an unprecedented degree of collaboration. The scientists then merged their results with those based on molecular data, an independent method of establishing evolutionary relationships, to develop a comprehensive new picture of the placental branch of life.

The break up of Gondwana. Credit: USGS

While their results support many previously known relationships, they reported some notably unexpected findings. For instance, the ancestors of elephants seem to have originated in the Americas even though they now inhabit the continents of Africa and Asia. This may not surprise you, since South America used to fit snugly into the curve of West Africa around 150 million years ago as part of the supercontinent, Gondwana. However, in the most controversial conclusion of this paper, the authors argue that this common ancestor did not evolve until after a catastrophic meteorite impact ended the dinosaurs’ reign around 65 million years ago. This means that our scurrying ancestor had to disperse around the globe after the break up of Gondwana, covering some serious ground and maybe even vast oceans.

Like most exciting science, this study raises almost as many questions as it answers. However, one thing is undeniably clear: Darwin couldn’’t have asked for a better birthday present.

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Check out Berkeley’s great resource on evolution and phylogenetic trees!

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Julia Rosen

Written by

Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelor’s degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her “Master's” in backpacking around the world and skiing. Julia is a periodic contributor to Oregon State’s research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.