Scientists draw tree to trace mammal evolution
John Wible and Michelle Spaulding participated in creating the Tree of Life, which includes DNA and anatomical descriptions of placental mammals. Scientists hope the Tree of Life helps enter details of fossils or animals they discover and determine where it fits into the Tree of Life, including what family of animals it is associated with and even when it most likely existed.
An artist's rendering of the hypothetical placental ancestor, a small insect-eating animal. The research team reconstructed the anatomy of the animal by mapping traits onto the evolutionary tree most strongly supported by the combined phenomic and genomic data and comparing the features in placental mammals with those seen in their closest relatives.
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Where each placental mammal exists on the evolutionary tree and what common ancestor they share are questions that have puzzled evolutionary biologists ever since Darwin.
But a study and database, funded through the National Science Foundation's Assembling the Tree of Life Program, already are offering answers about the evolution of placental mammals, humans included. The research not only describes the hypothetical common ancestor of all placental mammals, but provides a method of placing each species at its most appropriate place on the tree.
The online database at www.MorphoBank.org is a matrix of information that could bring advances in the science of mammal evolution much the way the decoded human genome helped to explain human biology. One scientist compared its importance to evolutionary science to the Large Hadron Collider's impact on particle physics.
Twenty-three scientists, including two from the Carnegie Museum of Natural History, worked for six years on the study and tree of life published today in the journal Science. The most comprehensive picture of mammal evolution to date combines 4,500 anatomical characteristics of living and ancient fossil mammals along with DNA from living mammals to produce an information matrix to help researchers untangle mammal evolution over the past 65 million years, and possibly farther back.
"We built an amazing tool to answer questions now, but this represents a building block for the future," said John R. Wible, the study co-author who holds a doctorate in anatomy and is curator of mammals at the Carnegie Museum of Natural History. "We built a framework and will build more and more on that framework as we add more fossils and more DNA."
Michelle Spaulding, a Carnegie Museum postdoctoral fellow with a doctorate in earth and environmental science, also participated in the study.
Placental mammals give birth to live babies, nourish offspring with milk, and have hair (fur). Yes, hippopotami and whales lack hair, but their ancestors once had hair, but it later disappeared, providing the animals the evolutionary advantage of baldness in watery habitats, much as swimmers shave their bodies and wear swimming caps.
There are 5,100 living placental mammals, only five species of egg-laying mammals and 310 marsupial species, including such pouched mammals as opossum and kangaroo. The placenta has the same membranes that surround embryos in reptile and bird eggs.
In the field of mammal biology, disagreements arose between the molecular group that uses DNA analysis to track evolution, and the morphology group that traces evolution through gradual anatomical changes through time. Mammal families known as clades typically are grouped according to similar anatomical characteristics, including bone structures and the size and shape of skulls, teeth, eyes, internal organs and feet, and whether they have fingernails, claws or hooves.
The molecular group generally has claimed the first placental mammal most likely lived 80 million to 100 million years ago, along with dinosaurs. But the morphology group seems to have won the argument, based on the matrix. They generally claimed that the common ancestor lived shortly after the Cretaceous-Paleogene extinction event 65 million years ago, when there were massive global extinctions, including dinosaurs, due in large part to an asteroid collision with Earth. Afterward, placental mammals emerged and quickly radiated worldwide.
If a new fossil is found, the researcher can indicate yes or no for 4,500 characteristics and then let the matrix place it at the most fitting position on the evolutionary tree.
Using the new matrix, researchers have traced each mammal characteristic backward through time to project anatomical features of the common ancestor, for which no fossil yet has been found. The resulting description is an insect-eating shrew-like animal, larger than a mouse, with an extended snout, many conical teeth, brown fur and white-fur underbelly, and an unusually long, thick, furry tail.
Another early finding involved previous thought that the flying lemur represented our closest primate ancestor. But the matrix reveals that the flying lemur and tree shrew are more closely related than previously thought, suggesting that the common ancestor of the flying lemur and tree shrew, rather than either animal, evolved into humans and other primates.
David J. Archibald, professor emeritus of the biology, evolutionary biology program area of San Diego State University, said the project has given evolutionary biologists "a tool we can use."
"They have found what I and others have said: The explosive model for placental evolution did not come about until after the extinction of dinosaurs [about 65 million years ago]. I think that's the big finding. All orders we are familiar with, even if you are not trained in biology -- primates, elephants, whales -- all groups show up within 10 million years of the end of the age of dinosaurs."
First Published February 8, 2013 12:00 am