Evolution Of Land Mammals

Evolution of land animals

THE LARGEST genetic study ever performed to learn when land plants and fungi first appeared on the Earth has revealed a plausible biological cause for two major climate events: the Snowball Earth eras, when ice periodically covered the globe, and the era called the Cambrian Explosion, which produced the first fossils of almost all major categories of animals living today.

According to the authors of the study, Science, plants paved the way for the evolution of land animals by simultaneously increasing the percentage of oxygen in the Earth's atmosphere and decreasing the percentage of carbon dioxide, a powerful greenhouse gas.

"Our research shows that land plants and fungi evolved much earlier than previously thought--before the Snowball Earth and Cambrian Explosion events--suggesting their presence could have had a profound effect on the climate and the evolution of life on Earth," says Blair Hedges, an evolutionary biologist and leader of the Penn State research team that performed the study.

The researchers found that land plants had evolved on Earth by about 700 million years ago and land fungi by about 1,300 million years ago--much earlier than previous estimates of around 480 million years ago, which were based on the earliest fossils of those organisms.

Prior to this study, it was believed that Earth's landscape at that time was covered with barren rocks harboring nothing more than some bacteria and possibly some algae.

No undisputed fossils of the earliest land plants and fungi have been found in rocks formed during the Precambrian period, says Hedges, possibly because their primitive bodies were too soft to turn into fossils.The early appearance on the land of fungi and plants suggests their plausible role in both the mysterious lowering of the Earth's surface temperature during the series of Snowball Earth events roughly 750 million to 580 million years ago and the sudden appearance of many new species of fossil animals during the Cambrian Explosion era roughly 530 million years ago.

"Both the lowering of the Earth's surface temperature and the evolution of many new types of animals could result from a decrease in atmospheric carbon dioxide and a rise in oxygen caused by presence on land of lichen fungi and plants at this time, which our research suggests," Hedges says.

"An increase in land plant abundance may have occurred at the time just before the period known as the Cambrian Explosion, when the next Snowball Earth period failed to occur because temperatures did not get quite cold enough," Hedges says. "The plants conceivably boosted oxygen levels in the atmosphere high enough for animals to develop skeletons, grow larger, and diversify." Lichens are believed to have been the first fungi to team up with photosynthesizing organisms like cyanobacteria and green algae.

Lichens can live without rain for months, providing protection for photosynthesizing organisms, which produce oxygen and release it into the atmosphere. The researchers suggest that the pioneer lichen fungi, which produce acids strong enough to dissolve rocks, also could have helped to reduce carbon dioxide. When washed away by rainwater, calcium released from lichen-encrusted rocks eventually forms calcium carbonate limestone in the ocean, preventing the carbon atoms from forming the greenhouse gas, carbon dioxide, in the atmosphere.

Land plants also can lower levels of carbon dioxide in the atmosphere. They have molecules called lignins, which contain carbon but do not readily decompose.

After the plant