When and Where Life Began
Almost 600 million years ago, before the rapid evolution of life forms known as the Cambrian explosion, a community of seaweeds and worm-like animals lived in a quiet deep-water niche near what is now Lantian, a small village in south China. Then they simply died, leaving some 3,000 nearly pristine fossils preserved between beds of black shale deposited in oxygen-free and unbreathable waters. Scientists from the Chinese Academy of Sciences, Virginia Tech in the United States and Northwest University in Xi'an, China report the discovery of the fossils in this week's issue of the journal Nature. The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid change; and what it would imply about the origin and evolution of animals. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures left in Cambrian rocks. The Lantia discovery suggests a much part of the picture.
In addition to ancient versions of algae and worms, the Lantian biota--named for its location--included macrofossils with complex and puzzling structures. In all, scientists have identified some 15 species at the site.
The fossils suggest that structural diversification of macroscopic eukaryotes--the earliest versions of organisms with complex cell structures--may have occurred only tens of millions of years after the Snowball Earth event that ended 635 million years ago.
Snowball Earth refers to the hypothesis that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 million years ago. The geological community generally accepts this hypothesis because it best explains sedimentary deposits generally regarded as of glacial origin at tropical paleolatitudes and other otherwise enigmatic features in the geological record.
Opponents to the hypothesis contested the implications of the geological evidence for global glaciation, the geophysical feasibility of an ice- or slush-covered ocean, and the difficulty of escaping an all-frozen condition.
The presence of macroscopic eukaryotes in the highly organic-rich black shale suggests that, despite the overall oxygen-free conditions, brief oxygenation of the oceans did come and go, according to H. Richard Lane, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research.
By 635 million years ago, the snowball Earth event ended and the oceans were clear of ice. Perhaps, Xiao (one of the authors) says, "that prepared the ground for the evolution of complex eukaryotes."
The team examined the black shale rocks because, although they were laid down in less than optimal waters for oxygen-dependent organisms, "they are known to be able to preserve fossils very well," says Shuhai (another author).
"In most cases, dead organisms were washed in and preserved in black shales. In this case, we discovered fossils that were preserved in pristine condition--some seaweeds still rooted--where they had lived."
The conclusion that the environment would have been poisonous is derived from geochemical data, "but the bedding surfaces where these fossils were found represent moments of geologic time during which oxygen was available and conditions were favorable," says Xiao.
The research team suggests that the Lantian basin was largely without oxygen, but was punctuated by brief oxic episodes that were populated by complex new life forms. Those life forms were subsequently killed and preserved when the oxygen disappeared.
Proving that hypothesis awaits further study.
The rocks in the study region are deposited in layered beds. The nature of the rock changes subtly, and there are finer and finer layers that can be recognized within each bed.
For further information: http://www.physorg.com/news/2011-02-oldest-fossils-large-seaweeds-animals.html or http://www.nsf.gov/news/news_summ.jsp?org=NSF&cntn_id=118598&preview=false