Finding the Oldest Evidence of Life
Chris - Researchers have found evidence of the earliest animal life on Earth. It shows that complex life was actually flourishing here much earlier in the Earth's history than we previously thought. Dr Gordon Love who's at the University of California at Riverside has covered the chemical fingerprints of sponges which are thought to be the first forms of animal life. He found it in rocks that are up to 750-odd million years old. Hello, Gordon. Welcome to the Naked Scientists. What have you actually done?
Gordon - What we did was we looked through rocks in a certain line and we have a continuous sequence of rocks and an opportunity for looking at the geobiology of this time. We found a continuous 100 million year record of sponges, going from a time known as the Cryogenian which is the time associated with two immediate glacial episodes and extending right up into the early Cambrian period.
Chris - Where did you get these rocks from? How did you find them?
Gordon - We were fortunate we were working with an oil company in Oman, Petroleum Development in Oman. They're just currently in an area of the world where people are producing vast amounts of oil from rocks of this age. Through our contacts in the oil company we had access to very pristine material from drill cores. That became very important in having this stretch of time and also the fact that these were very thermally well-preserved rocks that we were dealing with.
Chris - If you're looking at the fossil record though, and this frustrated Charles Darwin, life appears to pop into existence about 540 million years ago because prior to then presumably there weren't enough animals with hard body parts that could be fossilised. You've presumably run into the same problem. How do you know you've got sponges going back 750 million years?
Gordon - These are one of a number of steroid structures that we detected. The precursor to these molecules, the sterols are very specific to a class of sponge called demosponges. Sponges make a wide range of really unusual natural products which really interest the medical field as well. But in this case despite about four decades of research precursors have never cropped up. These precursors have never cropped up in single-celled organisms.
Chris - So what you're saying is you've now been able to unlock from the rocks chemical finger prints, if you like; molecules made by those early sponges 750 million years ago that when the animals died were locked away in those rocks. Although the animals, the vestiges of them, physically are gone their chemical legacy lives on and that is what you can detect.
Gordon - Yes, I think that's right. This is an important factor that Darwin was puzzled about. The lack of information is that if you look at smaller and smaller scale then there is a tangible record for animals significantly pre-dating the Cambrian explosion.
Chris - Just tell us a bit about the technique to get these molecules out of the rocks. You get your rock sequence from the oil driller. How do you then get out of the rocks the chemicals from the sponges? How do you know which rocks they've come from and that they were actually made by the early sponges all that time ago?
Gordon - We've a couple of approaches. The conventional way in organic geochemistry is to powder the rock after you've cleaned or removed the outer parts. Do some sort of solvent extraction technique, concentrated up the steroid components in the fraction and analyse these by gas chromatography/mass spectrometry. We found a really abundant amount of these steroids and in all the different rock formations that we looked at. To confirm that we were actually, it wasn't just coming from migrated oils that reside in the younger part of the section, what we did was we isolated what is an organic polymer. The bulk of sedimentary organic matter is actually insoluble and it can't migrate anywhere and they make up a large macromolecule. We basically broke chemical bonds by using a technique where we heat up samples with high hydrogen gas pressure. When we looked again at the steroid composition we saw that again these sterols were abundant products. In that way we could more confidently equate the age of the markers with the age of the rocks with uranium-lead isotope dating.
Chris - So this shows you that those rocks of that age had complex animals like sponges living in them. Why do you think that they're there? What does this tell us about the origin of animal life around on Earth at that time?
Gordon - Yes, most of the rock is in a depositional environment that we're looking at is shallow, marine waters. I think originally the simplest animals like sponges would have colonised the sea floor on a shallow continental shelf. It could have been tens of millions of years later we thought they could pervade into deeper water environments. I think it's telling us that at this time in terms of the environment there was finite levels of dissolved oxygen near the sea floor - at least on the shallowest water environments.
Chris - Does it fit with what we understand was happening elsewhere on earth at the time? Usually when you see an explosion of some evolutionary process there's something going on in the climate or in the Earth's other processes. Can you marry this observation with anything else going on at the same time that might explain what you're seeing?
Gordon - Yeah, now we've pushed this back, the first appearance, into the time frame of two vast glaciation events. My feeling on this is I go to many rocks of age which are older than the first glacial event and I have never seen any convincing evidence for sponge biomarkers. I think that these major glacial events radically altered ocean chemistry in the aftermath by shutting down a lot of obscure interaction. It seems, although we're still trying to get a handle on how exactly the chemical composition changed it seems they opened up avenues for new niches, especially for new organisms which could filter feed on the sea floor.