Australian scientists say they have found evidence that complex forms of life existed on the Earth 2.7-billion years ago, 500-million to 1-billion years earlier than previously thought.
The life forms are single-celled creatures called eukaryotes, the first known cells to have nuclei and specialized internal structures for processing energy. Present-day descendants of this group include all the higher kingdoms of life, like plants, animals (including people) and fungi, as well as many single-celled creatures like amoebae.
The first life, simple bacteria with no nuclei, is thought on the basis of fossils to have appeared by 3.5-billion years ago and possibly as long ago as 3.8-billion years ago. The Earth is about 4.6-billion years old.
The findings, reported in today's issue of the journal Science, are based on fatty molecules known to be produced by living cells that were found in ancient shale in northwestern Australia by a team led by Jochen J. Brocks, a geoscientist at the University of Sydney. The chemicals, called sterols, are found in the membranes of eukaryotes. Cholesterol is a member of this group of compounds.
Although it is possible that simpler organisms produced the sterols, the Australian researchers wrote in Science, the abundance of this class of chemicals "is convincing evidence for the presence of eukaryotes" around 2.7-billion years ago.
Other scientists noted that it was not known whether the cells that produced the fatty molecules had yet developed the other characteristics of eukaryotes. Besides the nucleus, these included small structures called chloroplasts, which enable plants to draw energy from the sun through photosynthesis, and mitochondria, which enable plant and animal cells to process energy.
"All we can say is that one important attribute of eukaryotes was in place" by 2.7 billion years ago, said Andrew H. Knoll, a paleontologist at the Harvard University Botanical Museum. In a commentary in Science, he wrote, "We can make only limited inferences about the overall biology" of the organism in question.
Nevertheless, he wrote, the finding highlights two points. First, there was a very long interval _ 1.5-billion years or more, if eukaryotes did arise as long ago as the new study suggested _ between their first appearance and the beginning of their flowering into higher organisms some 1.2-billion to 1-billion years ago.
Second, the findings push back still further the frontier of investigation of early life. For generations, Knoll wrote, an explosion of higher forms of life about 544-million years ago, at the beginning of the Cambrian period, marked a Terra Incognita beyond which little or nothing was known about life.