Christopher A. Mullin, professor of insect toxicology, holds a petri dish containing
corn rootworms. He is working on ways to use insects' sense of taste to control them.
Photo: Greg Grieco
By Barbara K. Kennedy
Eberly College of Science
Overwhelming evidence from the largest evolutionary study of gene sequences ever performed shows that the major groups of mammals emerged well before the extinction of the dinosaurs, according to researchers Sudhir Kumar, postdoctoral fellow, and S. Blair Hedges, associate professor of biology.
"The evolution of mammals appears to have occurred gradually by the isolation of breeding groups when the continents broke apart, not suddenly by the rapid filling of ecological niches left vacant when the dinosaurs became extinct," Hedges said. The massive gene study suggests that modern orders of mammals first evolved when the continents were separating during the Cretaceous era about 100 million years ago -- much earlier than some previous estimates based on fossil studies, which link the evolutionary event to mass extinctions 65 million years ago.
"This is the first time we ever have been able to estimate when all these lifeforms appeared on Earth," Hedges said. "Fossils can't give us this information, partly because there are huge periods of Earth's history from which not enough fossils have been found to make reliable estimates."
The scientists sifted through many thousands of vertebrate gene sequences from hundreds of species to find those that develop mutations at a constant rate over time, which Kumar and Hedges used like the ticking of a molecular clock to trace the history of each species back to its origin.
The researchers found that their molecular clock yielded origin dates similar to those based on fossil dating for many species, but for others the genetic clues lead back to a much earlier time.
Very few fossils resembling modern mammals or other vertebrates have been found in rocks from the Cretaceous period, said Hedges, partly because paleontologists rarely look for mammals in rocks that old.
"There has not been enough convincing evidence until now for paleontologists to invest their time and money looking for mammal fossils in a time before the dinosaurs became extinct," Hedges said.
Hedges said he hopes paleontologists will now begin searching for mammals in geological strata where they have never looked before.
"We are saying mammals definitely were living on Earth during the Cretaceous period from 70 to 100 million years ago. We don't yet know what they look like, but from the genes of their descendants we now know that they were there."
Christopher A. Mullin, professor of insect toxicology, holds a petri dish
containing
corn rootworms. He is working on ways to use insects' sense of taste to
control them.
Photo: Greg Grieco
By A'ndrea Elyse Messer
Public Information
Insects are probably more finicky than cats when it comes to their diets, so an insect toxicologist is targeting their taste buds in an effort to protect crops.
Christopher A. Mullin, professor of insect toxicology in the College of Agricultural Sciences, is working with corn rootworm to develop better methods of controlling this insect. Corn rootworm is the primary pest of corn in the United States. While its larva does most of the damage, this occurs underground and it is easier to target the adult beetle.
"Most pesticides are neurotoxins and must make it past all of the insect's defenses before they can effect the central nervous system," said Mullin. "Usually, hundreds of times more pesticide must be applied outside the insect so that a lethal dose can arrive at the proper location inside the insect."
To circumvent this problem, Mullin is looking at insect taste buds because they are a direct connection between the central nervous system and the outside. Insects also are specialty feeders, targeting one or two plant types as their primary food and starving if those plants are unavailable. Taste is very critical to insects.
Mullin does not know what insects perceive as taste, but he does know that chemicals considered sweet-tasting by humans are chemicals that stimulate feeding and, for the most part, chemicals that humans consider bitter-tasting put the beetles off their food.
One exception is cucurbitacin, an extremely bitter chemical found in the fruit skins and roots of squashes and other cucurbits, plants in the gourd family. Adult beetles will feed on anything coated with cucurbitacin to the exclusion of other foods. Farmers currently mix ground-up squash rinds with pesticides to get the insects to ingest the pesticides.
"But purified cucurbitacins are too toxic to use and too water soluble to remain on the tiny pollen-like spheres suggested as a delivery method," Mullin said.
Mullin is using three-dimensional chemical modeling to create structural templates of chemicals that stimulate feeding and those that suppress it.
Although chemicals that stimulate feeding behavior can be used to convince insects to consume poisons, Mullin would rather find a chemical that depresses feeding to the point of starvation. Corn rootworm beetles must feed to reproduce.
"While humans have thousands of taste buds, insects generally have four taste buds with four hairs per taste bud, or about 16 neurons," said Mullin. "Corn rootworm beetles have slightly more neurons."
The scientist is looking for a chemical that so overwhelms the taste bud neurons that they shut down, and because there are so few neurons, the insects would be incapable of perceiving their food and starve.
Newly discovered African fossils could resolve questions over the age and evolution of a species thought to be the most ancient known upright-walking ancestor of humans, according to research to be published today in the journal Nature.
The research team includes Meave G. Leakey, curator of paleontology at the National Museums of Kenya; Craig S. Feibel, assistant professor of anthropology at Rutgers University; Ian McDougall, professor of earth sciences at the Australian National University; Carol Ward, assistant professor of anthropology at the University of Missouri; and Alan C. Walker, distinguished professor of anthropology and biology at Penn State.
The researchers, who first named the new species Australopithecus anamensis in a paper published in Nature in 1995 based on 22 fossils discovered in northern Kenya, now have unearthed 38 additional fossils that paint a more complete picture of the species. Some scientists questioned the antiquity of the fossil bones described in the 1995 paper, because a definite time of origin could not then be measured.
"In 1995 we could not firmly establish the age of the youngest geologic layer associated with these fossils because the best dating technique needs crystals and this particular layer is mostly powdery ash," Walker said. "Since then we have managed to get enough good crystals to determine quite firmly that these fossils are between 4.1 million and 4.2 million years old." "We also have discovered 38 more fossils at this site that clearly show us how very primitive this species was."
According to Walker, the new fossils reveal that the ancient species had primitive jaws shaped more like a chimpanzee's than like a modern human's. It also had another characteristically primitive feature known as sexual dimorphism -- large differences between the sexes in overall body size and the shape of certain body parts, such as teeth.
Although some scientists believe the human family tree is bushy at its base, with multiple species evolving at the same time, Walker said all the early fossils discovered so far support a more tree-like picture of evolution for hominids -- the ancient ancestors on the human family tree.