Daily University Science News

A new study offers evidence that scientists are close to pinning down the evolutionary road map for placental mammals, the most diverse and largest subgroup of mammals.

This feat may resolve a long-standing scientific debate and shed light on the newly completed human genome.

The paper, published in Science today, was prepared by a team of University of Florida faculty members, graduate and undergraduate students, It follows publication in February of two related papers in Nature. Although authors in each case used different methods, their conclusions are remarkably similar, pointing to growing consensus on a topic that has long divided scientists.

"Five years ago, nobody thought this would happen," said Michael Miyamoto, UF professor and associate chairman of zoology and an author of the Science paper. "We have come from great pessimism to great hope that we've nearly resolved placental mammal history."

Scientists have been trying to sort out how animals are related to one another since Carolus Linnaeus, an 18th-century botanist and explorer, first came up with the basic principles for dividing them into species and larger groups.

Evolutionary biologists are interested not only in how to group animals, but also in which ones evolved first and how their traits are carried on or modified in animals that evolved later.

The traditional method is to compare the physical features of animals with other animals or fossilized specimens. Shared features -- anything from an animal's appearance to the shape and placement of its teeth -- form the basis for placing the animal within a species or the successively more inclusive groups of genera, families and orders.

Soon after the description of DNA as a double helix in the 1950s, scientists started using a completely new method to sort out such relationships: molecular genetics. This "molecular method" is similar to the traditional "morphological method" in that it makes decisions based on similarities and differences. But the molecular method's data are DNA and protein sequences.

The morphological and molecular methods have sometimes yielded different results, leading to an often acrimonious debate among scientists about which one is more accurate. The debate has flared up repeatedly over the years as arguments have occurred around certain animals or groups of animals, such as how closely rodents and rabbits are related.

The UF study took a step back. Instead of exploring the evolutionary history of this or that mammal or mammal group using one method or another, it sought to address the question for all placental mammals using both methods.

Mammals as a group are defined as milk-producing animals, with placental mammals comprising the largest group and the commonly recognized mammals, such as hoofed species, cats, primates and man. The other, much smaller, groups are marsupials, which include kangaroos and wallabies, and monotremes, the egg-laying mammals, which include the platypus and the echidna.

Rather than working in the lab or field, the UF researchers used the library and the Internet. In collaboration with Michele Tennant, assistant university librarian in the UF Health Science Center Libraries, the researchers identified 16,102 papers dating back to 1966 on placental mammal evolution.

In more than two years and thousands of hours, the team then narrowed the field to 1,477 papers that seemed to offer particularly promising data. Among this group, they found 315 publications that contained 430 published "trees," or graphic representations of evolutionary histories, compiled for different placental mammals with each method.

"We simply sought to collect as much data as possible," said Fu-Guo Robert Liu, the lead author of the paper and a UF doctoral student in zoology.

The researchers used a computer program to combine all the trees, cutting out differences and combining similarities. The result was two evolutionary "supertrees," one resulting from morphological research and the other from the molecular research.

The researchers noted first that the trees were quite similar through the broadly encompassing classification of orders. In defining how the orders are related, however, the trees diverge considerably, suggesting that future researchers should focus their efforts at this very high level.

"Our study narrows the scope of the problem," Miyamoto said.

The study further suggests that the molecular method may be better for resolving the relationships of orders -- but not in every case. To the contrary, the researchers found, where the trees disagree in the placement of the orders, the morphological method suggested the accepted solution about 20 percent of the time. Morphological evolutionists, for example, long argued that the orders for rodents and rabbits were closely related, a view only recently adopted by molecular evolutionists.

"There is a molecular bias against morphology, and I think this puts in better perspective what the values or merits of both can be," Miyamoto said.

The repercussions of the paper will go beyond evolutionists' circles, the researchers said.

By clarifying the evolutionary histories of other mammals, the placental mammal history will help scientists interpret the vast bulk of data generated in the human genome project. It will shed light, for example, on where people might have obtained certain genes and what role they played in their source animal ancestors.

"The tree is, in a sense, family history," Miyamoto said. "Doctors ask about family history to better understand your health. Biologists do the same to better understand genes and physical traits of humans, not to mention domestic animals and endangered species."

The Science paper also may prove useful in the emerging field of genomic "prospecting." In this field, scientists hunt for genetic clues to how living animals survived past catastrophes, such as disease epidemics, with the end goal of one day using the clues to help develop new and better strategies for human health and medicine.

"The bottom line is that knowledge about your brothers and sisters provides insights about you," Miyamoto said.

Other researchers who participated in the study are Nicole Freire, a master's student in zoology; Phong Q. Ong, a former undergraduate in zoology; Timothy Young, a doctoral student in zoology, and Kikumi Gugel, a former undergraduate in zoology.