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Question: When did the first living creatures appear on Earth?
The oldest shelled animals to leave their hard parts behind in the geologic record as fossils are mollusks, relatives of modern snails. These first shells are found at the base of the Paleozoic Era in rocks, which date from the Cambrian Period, and thus are about 530 million years old. Actually, we define the base of the Paleozoic Era (Time of Ancient Life) as the oldest deposits with fossilized shells of marine animals preserved there.
The oldest fossils of multicellular animals that we have come from South Australia, from an area called the Ediacara Hills, and are impressions in ancient shallow water sands now cemented to form sandstones, approximately 700 million years in age. Although the Ediacaran fauna did not have hard parts to be fossilized, many more modern forms of marine (salt-water) animals trace their ancestry to them. We also have fossils of single celled organisms from still older rocks. In fact, we have very consistent and reliably dated fossils of bacterial deposits as old as 3.6 billion years. Enough such fossils have been found to convince everyone that they are, in truth, remains of single celled organisms, some better organized, with a nucleus in the cell, and others lacking it. It is these more poorly organized single celled organisms that occur in the oldest rocks.
Now there are geologists, chemists and biologists looking at yet older rocks and the occurrence of carbon in them, and have shown that this carbon is very similar in its character to organic carbon (its isotopic composition in chemical terms). This is an element present in all organisms known to inhabit or to have inhabited the earth. If this research progresses as expected, we may soon have a yet older date for the appearance of the first living creatures, conceivably as old as 4 billion years. Certainly we do know that the 3.6 billion year old bacterial, single celled fossils were not the most primitive or oldest organisms to live on earth.
Regarding the dating of rocks and fossils; I have used terms such as "million or billion years before the present", and these refer to ages that have been determined using radioactive dating. There are a number of radioactive elements that break down, or decay, at known rates. Since the 1970's we have had instruments to measure the very small amounts of these that occur naturally in some rocks. We can determine the rate of decay of radioactive elements, measure their amounts in rocks, and use these to determine the age that the rock was formed. These dates are what we use when we express the age of events in earth history in terms of years before the present.
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