A recent Nature publication reports a new technique for measuring the oxygen levels in Earth’s atmosphere some 4.4 billion years ago. The authors found that by studying cerium oxidation states in zircon, a compound formed from volcanic magma, they could ascertain the oxidation levels in the early earth. Their findings suggest that the early Earth’s oxygen levels were very close to current levels.
Organic reactions are reactions that involve carbon-containing compounds. For the purposes of origin-of-life research, we are interested in nucleotides, the organic compounds that comprise DNA and RNA, or amino acids, the organic compounds that are the building blocks of proteins. Among origin-of-life scientists, the RNA-first world hypothesis has gained much ground. (See here for an article about recent research in this area.) However, RNA comes from other RNA molecules, and proteins come from other proteins. So the first molecules of life must have arisen by another process, different from how they are formed today. Miller and Urey conducted experiments to show that under certain atmospheric conditions and with the right kind of electrical charge, several amino acids could form from inorganic compounds such as methane, ammonia, and water. Several experiments have been done using various inorganic starting materials, all yielding a few amino acids; however, one key aspect of all of these experiments was the lack of oxygen.
If the atmosphere has oxygen (or other oxidants) in it, then it is an oxidizing atmosphere. If the atmosphere lacks oxygen, then it is either inert or a reducing atmosphere. Think of a metal that has been left outside, maybe a piece of iron. That metal will eventually rust. Rusting is the result of the metal being oxidized. With organic reactions, such as the ones that produce amino acids, it is very important that no oxygen be present, or it will quench the reaction. Scientists, therefore, concluded that the early Earth must have been a reducing environment when life first formed (or the building blocks of life first formed) because that was the best environment for producing amino acids. The atmosphere eventually accumulated oxygen, but life did not form in an oxidative environment.
The problem with this hypothesis is that it is based on the assumption that organic life must have formed from inorganic materials. That is why the early Earth must have been a reducing atmosphere. Research has been accumulating for more than thirty years, however, suggesting that the early Earth likely did have oxygen present. Unfortunately, the idea that inorganic compounds can come together to eventually form the building blocks of life was such a good story that the Miller-Urey experiment and the assumption about the early Earth’s reducing atmosphere were perpetuated in the classroom long after the scientific evidence had turned against them. (See here for an article on misuses of the Miller-Urey experiment.)
This brings us back to that Nature letter, which offers another reason to question the reducing environment of the early Earth. The authors looked at the cerium oxidation levels within zircon. Zircon is a hard rock that forms from the solidification of magma. Several studies have been done with zircon because of its age and durability. (See here for a report on using zircon to determine when oceans and land were present on Earth.) Cerium (Ce) is an element that can be found in early Earth zircon. The ratio of cerium’s +3 and +4 oxidation states indicates the environmental conditions at the time the cerium was trapped in the zircon. Specifically, cerium’s oxidation ratio is related to the amount of oxygen in the atmosphere. The authors created cerium-infused zircon in the lab at various oxidative ratios to make a calibration curve to which they could then compare the early Earth samples.
Their findings not only showed that oxygen was present in the early Earth atmosphere, something that has been shown in other studies, but that oxygen was present as early as 4.4 billion years ago. This takes the window of time available for life to have begun, by an origin-of-life scenario like the RNA-first world, and reduces it to an incredibly short amount of time. Several factors need to coincide in order for nucleotides or amino acids to form from purely naturalistic circumstances (chance and chemistry). The specific conditions required already made purely naturalist origin-of-life scenarios highly unlikely. Drastically reducing the amount of time available, adding that to the other conditions needing to be fulfilled, makes the RNA world hypothesis or a Miller-Urey-like synthesis of amino acids simply impossible.
The study clearly presupposes a certain confidence level in zircon dating. Yet our confidence on that point is open to debate. Certainly zircon seems to exhibit toughness and durability. However, some scientists question zircon dating because it is based on uranium and lead levels within zircon. Others argue that zircon-dating techniques are quite good and that these zircon samples are the oldest samples we have. The nature of the debate and the specifics about zircon-dating methods are beyond the scope of this article, but the issue merits mentioning because it affects the implication of the results of this research.
What does it all mean for origin-of-life research? In an interview, one of the authors of the paper, Bruce Watson, states that “We can now say with some certainty that many scientists studying the origins of life on Earth simply picked the wrong atmosphere.” He hopes that the results of their research “may reinvigorate theories that perhaps those building blocks for life were not created on Earth, but delivered from elsewhere in the galaxy.” Perhaps. But just because we are having trouble showing that life arose from purely naturalistic causes on Earth, it doesn’t necessarily follow that the solution must be found elsewhere.