Looking at molecular fossils allows scientists to peer back into the earliest history of life. Large fossils like imprints in rocks no longer survive from this time period, so scientists must study the chemical traces left behind by these ancient animals. “It seems to be the method that shows the most promise for showing the true origins of animals,” says Andrew Knoll, a paleontologist at Harvard University.
Scientists have been using molecular fossils for this purpose for only the last few years. But oil companies have studied and used molecular fossils for several decades. These companies use molecular fossils to identify pools of oil discovered by drilling miles underground. It’s a lot like fingerprinting a person to find out who they are.
Every pool of oil has its own unique mix of molecular fossils, because every pool of oil came from different sources. Some oil came from plants, animals and bacteria that once lived in a river valley or jungle; other oil formed from dead things that drifted to the bottom of the sea. And different pools of oil formed at different times in history. Some oil is 100 million years old; some of the oil in Oman is up to 751 million years old. Because each pool of oil formed in a different time and place, each pool of oil formed from a
different set of living things — different kinds of plants, animals and bacteria.
When geologists find oil in two different wells, they can test the molecular fossils in both places to see whether the oil comes from the same source miles underground. Knowing whether or not two wells share the same source helps scientists and oil companies decide where and how to explore for more oil. Molecular fossils have even been used to identify the source of an industrial oil spill that washes up on a beach.
In fact, Gordon Love and Roger Summons’ discovery of the first animals began with a business trip. Love went to Oman to work for an oil company for what he thought would be a few days. The company wanted him and Summons to compare molecular fossils in a pool of oil with molecular fossils in rock cores found by drilling. By doing this, scientists could figure out which rocks the oil drained out of millions of years ago.
But while Love and Summons were working, they became interested in the 24-isopropylcholestane (24-ipc) that they saw in some of the rocks. They saw this chemical as a possible sign of ancient sea sponges. “We didn’t start the project to look for sponges” or 24-ipc, says Love. “But it became apparent that it this was an abundant signal in our samples. Then we wondered how far back in time these things went.” By tracing that molecular fossil back in time, Love and Summons found the oldest known fossil evidence of animals on Earth.