"Our study is the first to provide statistically unambiguous results in favor of classifying mammals using the Theria hypothesis, as paleontologists have long done through studying fossils," Jirtle said. "Now we need to retest the results generated by scientists who have used mitochondrial DNA sequences to link mammals such as hippos to whales."
The Duke scientists generated their results by isolating a whole nuclear gene from the genetic material of 15 different mammals, and then determining the unique genetic code or sequence that distinguishes each gene from the others in the respective mammals.
Then, by plugging molecular traits of the gene into a computer software program -- similar to entering eye color and earlobe structure into a family tree software program -- the scientists identified which animals shared common DNA traits and which did not. The data they derived from studying nuclear genes clearly identified marsupials (kangaroos, opossums, etc.) as having a common evolutionary background with eutherians (humans, pigs, etc.), and monotremes (platypus, echidna) as having evolved separately. Jirtle said these conclusions contradict the highly publicized assertions Ulfur Arnason and colleagues reported in a 1997 article in the journal Proceedings of the National Academy of Science.
Aside from its purely academic value, the scientists said that classifying mammals correctly is critical because it helps biologists apply the information learned from one mammal to others in the same class, without having to conduct identical molecular studies on each mammal.