McKenna is essentially studying that process in substances like myoglobin, which stores oxygen in muscles. By interpreting similarities in the kinds of myoglobin used by three animals, for example, he tries to figure out which two animals have a more recent common ancestor than they both share with the third. From such studies, family trees can be constructed.
Fossil Record Checks
It's not easy. One of McKenna's current analyses, for example, deals with about 60 animals, including fish, alligators, chickens, penguins, the duckbill platypus, dogs, horses, sheep, rabbits, two kinds of elephants and man. He is checking the results of the analysis against the fossil record to see if they say the same things about the family tree.
Fossils give their own insights, sometimes unexpected, into how ancient animals were related. When McKenna examined a jaw unearthed recently in Washington state by a Smithsonian Institution colleague, he agreed that the unknown animal was related to elephants. But the jaw had been found in marine sediments, suggesting that it came from an oceangoing beast.
"Suddenly, it dawned on us," McKenna said. The jaw was from an early order of mammals called desmostylians, which were considered close relatives of the manatees and dugongs that sailors believed were mermaids. Further research by McKenna, Daryl Domning of Howard University and Clayton Ray of the Smithsonian Institution showed that the beasts were actually more closely related to elephants than to dugongs and manatees.
Teeth Were Key Factor
The tip-off was the teeth, which looked clearly elephant-like 35 million years ago when the fossil jaw was formed but lost that character about 10 million to 15 million years ago, the period of previous desmostylian fossil finds.
McKenna did not find the key fossil but, he said: "I had part of the joy of figuring out what it is." He called that one of the "levels of discovery" for fossil finds, beyond just finding the fossil and recognizing what beast it came from.
Standing in one of the huge rooms where the museum stores fossils, McKenna described "the fun of following your nose" to see what significance a fossil might have.
"There are specimens all around you here that we have no idea of what they mean," he said. They may be keys to solving a scientific problem, and "we might not even know that problem exists."
Science appeals to McKenna because "it is so rich in different approaches and different subjects that can be brought to bear on central problems. . . . There's an awful lot of enjoyment in bringing different approaches to bear on a given problem, to do detective work. Doing the detective work is just plain fun.
"Good science is not dogmatic. It's open-minded stuff," he said. It is "a testing and therefore humble activity. In a sense, we are willing to admit we are wrong."
McKenna, for example, did that after he published a paper in 1975 addressing why five premolar teeth had been observed in mammals of 80 million years ago, while only four are seen in modern-day placental mammals. He wrote that some common ancestor apparently made the switch, which was inherited by later generations.
But fossils then showed that a fairly advanced group of mammals still had five premolars. McKenna now theorizes that different groups of mammals lost the fifth tooth independently rather than through a common ancestor.
Theory, contrary evidence, new theory, new contrary evidence.
"You never necessarily get to the end of this," McKenna said.