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 is offering early answers about the evolution of placental mammals, humans included. It also describes a hypothetical common ancestor of all placental mammals, along with a method to place each species at the right place on the evolutionary tree.
The online database is a matrix of information that could advance the science of mammal evolution, much as the decoding of the human genome helped to explain human biology, biologists say.
Researchers build 'Tree of Life' for mammals
Two local researchers at Carnegie Museum of Natural History are among 23 scientists around the world unveiling the "Tree of Life" database that shows where each placental mammal exists on the evolutionary tree. (2/8/2013)
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 post-doctoral fellow with a doctorate in earth and environmental science, also participates in the study.
The online database is available at www.MorphoBank.org.
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 that disappeared due to evolutionary advantages of baldness in watery habitats, much like swimmers shaving their bodies and wearing 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 the opossum and kangaroo. The placenta has the same membranes that surround embryos in reptile and bird eggs.
In the field of mammal biology, disagreements emerged between the molecular group that uses DNA analysis to track evolution, and the morphology group that traces evolution through gradual anatomical changes.
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 the dinosaurs. But the morphology group appears to have been more accurate, based on the matrix. They generally claimed that the common ancestor lived after dinosaurs became extinct 65 million years ago, then quickly radiated world wide.
Using the new matrix, researchers developed the profile of the common ancestor of all placental mammals, since fossils of such an animal have yet to be discovered. Their conclusion is a shrew-like animal with an extended snout, many conical teeth, brown fur and white-fur underbelly, with a long, thick, handsome tail.
Another early finding involved previous thoughts that the flying lemur represented our closest primate ancestor. But the matrix reveals that the flying lemur and tree shrew are more closely related, leading to an alternative explanation that humans and other primates share a common ancestor with both the flying lemur and tree shrew.
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 7, 2013 7:45 PM