Molecular herbarium. Specific plant-leaf molecules preserved in the bottom muds of this Mexican lake recorded a shift toward grasses since the last ice age.

How Grasses Got the Upper Hand

Grasslands began to take over the tropics and subtropics some 7 million or 8 million years ago. Researchers thought they could explain the sudden ascendance, but loose ends keep appearing in the common explanation. Now a study in the 31 August issue of Science points to a new factor: A change in moisture levels might have been the key to grasses' success.

The common explanation for the rise of grasses is a slow dwindling of carbon dioxide in the atmosphere during the past 100 million years. Grasses thrive under lower CO2 concentrations in the atmosphere, but numerous other factors influence their abundance. Despite the uncertainty, the sudden prevalence of grasses might have encouraged the evolution of hoofed animals well adapted to graze on the new resource.

To track grasses' early proliferation, organic geochemist Yongsong Huang of Brown University in Providence, Rhode Island, and colleagues compared the ecological histories of land around two Mesoamerican lakes as atmospheric CO2 increased since the peak of the last ice age. From cores of lake-bottom mud, Huang and his colleagues isolated distinctive long-chain hydrocarbons derived from leaf wax of land plants washed or blown into the lake. By measuring the carbon isotopic composition of these hydrocarbons, they could gauge the relative proportions of land plants like shrubs and trees, which are at a disadvantage under low CO2, versus tropical grasses.

Around 18,000 years ago, they found, a relatively wet climate in the northern Mexico state of Chihuahua supported a predominance of shrubs and trees. But after the end of the last ice age, the region dried out, and grasses came to predominate. More than 2000 kilometers to the southeast, under the same declining CO2, the post-ice-age weather shift tended to make the region around Lake Quexil in Guatemala wetter rather than drier. There, shrubs came to predominate.

Huang says, "low CO2 was not enough. Aridity probably was also very important" in the rise of grasslands. Geochemist Jay Quade of the University of Arizona, Tucson, agrees that "CO2 is not the only explanation" for the ancient shift to grasses. Whether climate or CO2 is foremost "is a matter of emphasis," he says.