The History of Earliest Life

This segment is an account of life in the Precambrian, the interval of time extending from the formation of the Earth 4,600 million years ago to the beginning of the Cambrian Period about 545 million years ago. During this interval, representing 88% of the Earth's history, life originated, primitive single-celled organisms evolved into more complex ones, and finally, almost at the end of the interval, the first multi-celled organisms appeared. None of the organisms present in the Precambrian had developed hard skeletons or shells that would be readily preserved as fossils, and consequently their fossil record is very sparse.

The oldest known fossils occur in cherts (flint-like rocks) dated at about 3,500 million years old in South Africa and the Pilbara region of Western Australia. In both areas the fossils are of two types: firstly, layered sheet-like, dome-like or columnar structures called stromatolites, which are considered to be the trace fossils of bacterial communities; and secondly, the microscopic remains of the bacteria themselves in the form of carbonaceous spheres with an average diameter of about 0.0025 mm, and thread-like or filamentous structures. Similar fossils occur sporadically throughout the remainder of the Precambrian in Australia, Africa, India, North America, and Greenland, and closely resemble forms that are still alive today.

These fossils indicate that life had begun by the end of the first 1,000 million years after the formation of the Earth 4,600 million years ago, but they give us no information on the processes leading to the origin of life. There has been much speculation on how life may have originated, but it is generally believed to have developed in the oceans, from a 'hot dilute soup' of organic molecules. These were probably formed from inorganic substances including ammonia, methane and hydrogen by the action of some form of energy such as lightning, ultraviolet radiation, or heat from lava. This chemical process could only have taken place if there were little or no oxygen present in the primitive atmosphere.

The earliest organisms of which there is a fossil record are simple, primitive cells without a nucleus or other structures within them. By 1,700 million years ago another type of cell had appeared, larger in size and with a nucleus and other cell structures such as mitochondria. Nucleated cells of this type compose over 99% of organisms living today. Fossils of nucleated cells are known from the Bungle Bungle Dolomite of Western Australia, dated at about 1,500 million years old, and from the 800 million year old Bitter Springs Formation of central Australia. Nucleated cells require oxygen to produce energy for cell functions, so by the time they appeared, the Earth's atmospheric oxygen levels must have increased to a level sufficient to enable them to survive. The oxygen entering the atmosphere is believed to have originated as a by-product of photosynthesis carried out by some of the earliest, non-nucleated cells.

Development of nucleated cells was an essential precursor to the appearance of multi-celled organisms. These had appeared by 1,300 million years ago, because multi-celled algae of this age are known from the United States. Multi-celled animals, or metazoa, seem to have appeared somewhat later. The first evidence for them is found in rocks 900 million years old and consists of simple trace fossils suggesting burrowing activity; however, not all experts agree that these structures are indeed fossils.

The earliest body fossils of metazoans appear in rocks dated at between 580 and 650 million years old. These fossils are known as the Ediacaran fauna after the Ediacara Hills in the Flinders Ranges of South Australia, where the most famous and diverse assemblage of these organisms was found in 1947. Similar fossils are now known from other localities in Australia, as well as from South Africa, Europe, Asia, North America and possibly South America. The Ediacaran fauna consists of a variety of circular, leaf-like, frond-like or segmented organisms, all of them entirely soft-bodied and preserved as impressions in sandstones that were deposited in shallow marine environments. The biological relationships of the fauna are problematical. Some of the forms present have been interpreted as jelly fish, worms, soft corals, primitive echinoderms and primitive arthropods, but there is now evidence to suggest that some may belong to types of organisms no longer surviving at the present day. It is clear that the Ediacaran fauna does not represent the ancestors of the shelly faunas that were abundant in the succeeding Cambrian Period and the remainder of the Palaeozoic Era.

References:

Strother, P. K. 1989. Pre-metazoan life. In Allen, K. C. & Briggs, D. E. G. (eds), Evolution and the fossil record, Belhaven Press, London, 51-72.