Until recently, scientists believed that life on Earth did not emerge onto land until 1.2 billion years ago. In October 1999, Dr. Hiroshi Ohmoto of the NASA Astrobiology Institute pushed back that date a billion years by discovering 2.3 billion-year-old rock formations called laterites. Now, the discovery of 2.6 billion-year-old fossilized microbial mats intermixed with soil has pushed back that date even further.
A team of scientists discovered the fossils in core samples taken from a mine in South Africa, which contained a layer of iron-rich soil combined with fossilized microorganisms. This mat-like layer was sandwiched between layers of rock, the lower one dated 2.7 billion years old, and the upper one 2.6 billion years old.
"Therefore, the age of soil formation and of the development of soil was between 2.6 and 2.7 billion years, perhaps closer to 2.6 billion years," says Ohmoto.
The scientific team that made the discovery included Nick Beukes of Rand Afrikaans University, Johannesburg, South Africa; Yumiko Watanabe, a Ph.D. student at Penn State Univeristy; Ohmoto, who is professor of geosciences and director of the Penn State Astrobiology Research Center,; and other scientists.
Microbial mats (or biomats) are aggregates of microorganisms composed mainly of bacteria and algae. They can grow in a number of environments: shallow seas, lakes, ponds, rivers and on soils. Because soils form only on land, however, the presence of these microorganisms intermixed with soil means that life must have emerged from the sea onto the land at least 2.6 billion years ago.
|Laterites are layers of soil colored red from high concentrations of iron. This example is from Brazil. Credit: Univ. of Colorado/Joe Smyth.
This finding is surprising because it contradicts what many scientists believe about when in Earth's history the conditions arose that are considered necessary for life to have emerged onto land. It is generally agreed that life on land was not possible until oxygen had built up in the Earth's atmosphere to create a protective layer of ozone.
Ozone forms a protective screen against ultraviolet radiation, which can destroy land-based life. Many scientists believe that there wasn't enough oxygen in Earth's atmosphere to form an ozone layer until around 2.2 billion years ago, 400 million years later than the point in time when, according to Ohmoto, life was already flourishing on land.
When microbial mats develop on soil, they often lead to the formation of laterites, which are soil layers enriched in iron oxides. These layers form when living things decay. The resultant organic acids leach iron from upper layers of soil and deposit them as oxides in the soil layers below. This process creates three distinct bands of soil an iron-rich layer sandwiched between two iron-deficient layers.
Modern-day laterites form mostly in the tropics, where large amounts of organic material decay rapidly.
"The 2.6 billion-year-old fossilized biomats are not real laterites," Ohmoto points out, "but they occur together with the iron-rich minerals that represent an early stage of laterite formation."
Laterites provide direct evidence of an oxygen-rich atmosphere, but only indirect evidence of the presence of land-based organisms. "Some people may argue that the leaching of iron may occur by processes not related to organisms and their products," says Ohmoto.
Likewise, microbial mats on soil provide direct evidence for the presence of land-based life, but only indirect evidence of an atmosphere rich enough in oxygen to produce a protective ozone layer. Some researchers suggest that microbial mats may be able to develop on land without an ozone shield.
But, Ohmoto reasons, the combination of iron-rich minerals and fossilized microbial mats in rocks dated at 2.6 billion years old proves both that oxygen must have been present in the Earth's atmosphere and that life must have been present on land at that time.
"Of course, terrestrial life back then was more in the nature of bacterial mats than oak trees and mammals," says Ohmoto.
Ohmoto is not surprised to find evidence of land-based life so early in Earth's history.
"Like many other people, I was erroneously influenced by the concept accepted for the invasion of animals from the ocean to land," says Ohmoto. "That is, bacterial mats moved from coast to inland like the crawl of amphibious animals. However, I now believe that the transport of microbes from the ocean to land, and vice versa, has been carried out mostly by the wind: the splash of ocean waves containing microbes and soil containing microbes are transported globally by the wind."
Because of this process, Ohmoto believes that microbial communities on all the continents would have similar characteristics.
Microbial mats as old as 2.6 billion years can only be dated indirectly, by determining the age of the rocks positioned above and below them. "There is no way to directly date the fossil age of microbial mats older than a million years," says Ohmoto. "The ages can, however, be determined on soil minerals that host biomats, or on the rocks that sandwich the soil horizon."
Dating microbial mats and early laterites is not as difficult as finding them. According to Ohmoto, one of the biggest problems in the dating process is finding such soil layers still intact between two layers of datable rocks. Because the Earth has such an active geology, it is extremely difficult to find these sandwiched layers undisturbed.
|Animation: Earth's ozone layer protects land life from ultraviolet radiation. Highest concentrations of ozone are shown in red (Data from 1998-99). Credit: NASA.
"Because of the plate tectonics that cause subduction of crustal rocks into the mantle, the chance of finding older rocks, especially of those formed under a certain paleogeographical condition, diminishes exponentially" over time, says Ohmoto.
Ohmoto and his team have already begun investigations of 2.7 billion-year-old paleosols (ancient layers of buried soil) in Australia and 2.9 billion-year-old paleosols in Ontario, Canada. Ohmoto hopes eventually to reconstruct an accurate history of the evolution of atmospheric oxygen and its relationship to the evolution of biosphere.
"A long-term goal is to understand the connection between environmental evolution and biological evolution," he says.
Knowing the basic rules of evolution such as how species develop or how the environment impacts a species over time can help scientists study Precambrian fossils that have no known modern counterpart. Astrobiologists can also use this knowledge as a guide in the search for life on other worlds.
As we continue to search for life elsewhere in the Universe, new discoveries about the evolution of life on Earth will color our perceptions about astrobiology. While geologists still debate about exactly when significant amounts of oxygen appeared in the Earth's atmosphere, these laterites suggest that oxygen was plentiful at least 2.6 billion years ago. Ohmoto thinks investigations of rocks older than 3.5 billion years could yield even more answers to that question in the future.
Related Web Pages
Astrobiologists Find Evidence of Early Life on Land - NASA Astrobiology Institute
Ancient South African Soils Point To Early Terrestrial Life - Penn State
Geochemical Evidence for Terrestrial Ecosystems 2.6 Billion Years Ago - Nature (registration required).
Figures 1-4 - Images, graphs, and maps accompanying Nature article (registration not required).
The First Ice Age - Scientific American. Article from 1999 reporting Ohmoto's evidence of life on land 2.3 billion years ago.