Scientists studying the grasshopper mouse are hoping to learn more about how humans made the switch from being plant-eaters to predators.

Named for the insects that form a major part of its diet, the grasshopper mouse shares a rare trait with modern Homo sapiens: Each went from eating only plants to eating both plants and animals.

"We can come up with any number of theories to explain why the shift to predation" occurred, said Pat Shipman, a paleontologist with the Johns Hopkins University School of Medicine, in a presentation to the AAAS meeting. "But we have to remember that such a change would likely have large physiological consequences."

Shipman has focused on the ecological results of such a major change in dietary habits.

The animals in an ecosystem are often portrayed in the form of a pyramid. At the base, with the greatest numbers, are plants, which receive their essential nourishment directly from sunlight.

Above them in lesser numbers are the animals that eat plants. And above these animals in even fewer numbers are the animals that eat other animals.

To move up the ladder, a species must immediately reduce its population to have enough food. That means either that many individuals die, or they must be dispersed across more territory. The earliest species to spread out of Africa and migrate to Europe and Asia was Homo erectus, possibly as long as 1 million years ago.

The evolutionary decision to become a predator is complex. Animals are adapted to specific ways of eating. Most animals have teeth that have evolved to help them grind plants. Their stomachs are designed to help them digest a specific type of food.

Shipman said the grasshopper mouse began eating insects when it spread across the American Midwest and West in an evolutionary attempt to find more food. The change took centuries and parallels the manner in which early humans may have moved away from a diet of plants.

First they may have become scavengers, finding it easier to gather high-protein food from dead animals and learning when it would be safe to approach them. This type of eating is similar to the way plant-eaters seek food. The next step is hunting.