Everybody knows about dinosaurs and other fossils, but what do we believe were the first organisms on Earth — and why do we believe it?
A timeline for early Earth
Mainstream science currently believes that the Earth was formed about 4.54 billion years ago (Ga), and the oceans first formed about 4.4 Ga. The first age of Earth is called the Hadean, and it predates most of the surviving rocks today. We believe that a period known as the Late Heavy Bombardment took place from 4.1 to 3.8 billion years ago, and consisted of the Earth getting hammered by an intense meteorite and other impacts which may have delivered new chemicals to the surface. For a long time, scientists believed that the Earth was lifeless for billions of years, but now more and more evidence seems to be showing up to suggest otherwise.
What is abiogenesis?
Most scientists today support the idea of “abiogenesis” — meaning life from an unliving environment. The basic premise of abiogenesis is that all the basic chemicals needed to form life were muddled together, mixing and re-mixing until, by pure chance, some structure formed which was capable of sustaining and reproducing itself. This may sound far-fetched (and indeed most religious fundamentalists assert that it’s nonsense), but it might not be; the hydrocarbons that form amino acids and other molecules necessary for life are naturally inclined to stick together in increasingly complicated ways, some of which tend naturally to a chain reaction that would extend itself. Still, the theory has never been proven! Abiogenesis has so far been impossible to reproduce in a laboratory environment despite numerous attempts to speed up the process, and some of the steps are still pretty unclear. Now that we believe the earliest life on Earth may have shown up almost immediately after the oceans formed, it’s certain that there’s something we’re missing. Some evolutionary biologists believe life here may have been “seeded” by spores from a solar system that formed earlier. For now, it’s going to remain a topic to explore and debate.
Cyanobacteria in stromatolites
The name “cyanobacteria” refers to the blue-green color of these microbes. Like all the contenders for the earliest organism, cyanobacteria are “prokaryotes”, a very simple form of life that does not have a cell nucleus. As a byproduct of photosynthesis (turning light into energy for life), cyanobacteria produce oxygen. It’s believed that the earliest atmosphere on Earth had little or no oxygen and these little guys produced so much that it “rusted” the Earth, killing off most of their anaerobic rivals (to whom oxygen is poisonous). Rocks found in Greenland appear to show fossilized, primitive cyanobacteria “stromatolites,” structures with seemingly artificial layers. These fossils appear to be from about 3.7 Ga, or less than a billion years after the Earth was formed.
Microbial mats and other remnants
Scientists in Australia have found fossils that they say may be from a photosynthetic “microbial mat” from about 3.5 Ga. More Greenlanders have found 3.8 Ga rocks that show nuclear isotopes they think indicate metabolic activity — nothing else is known about what this microbe might have been. And microfossils in Canada, which look like filaments and show signs of biological chemicals, might be as old as 3.8 – 4.29 Ga! If this is verified, it would mean that life formed almost immediately after the oceans, which would be remarkable — a lot of scientists are scratching their heads to figure out how that could be true. One group of scientists has also found evidence that carbon isotopes in graphite indicate life at 4.1 Ga. These last two claims imply that life might have arisen at the same time as the Heavy Bombardment — could they be linked?
Archaea at the bottom of the ocean
A final approach to answering this question was to use biology rather than fossil hunting and analyzing isotopes. A team of researchers investigating the genetics and other chemicals common to early and simple organisms tried to find their “LUCA” or Last Universal Common Ancestor (not entirely unlike the attempts to find the original “Eve” or mother of the human race by examining mitochondrial DNA). They compared proteins shared by all bacterial and archaeal organisms and reverse-engineered a model of LUCA’s DNA. What they found was that LUCA seems to have had some physical characteristics shared by modern hyperthermophilic lifeforms, which today live near volcanic vents at the bottom of the ocean. “Archaea” are another form of prokaryote which were once believed to be an ancient form of bacteria, but are now known to actually be similar to eukaryotes (lifeforms with cell nuclei) in many ways, and belong to their own kingdom. These odd lifeforms live in many places around the world, including near undersea volcanic vents (along with some kinds of bacteria). While most life on Earth breathes oxygen and gains energy derived from sunlight, organisms near these vents can be entirely anaerobic and survive on energy they gain from the heat and chemicals found there spilling out of the Earth’s crust. We still don’t know as much about these critters as we do other life, for the obvious reason that it’s just not that easy to get down there and explore. The LUCA experiment seems to show that the first organisms on Earth were similar, at least in proteins, to these lifeforms — which may or may not mean they are descendants.
If life was around as early as the oldest rocks we can find, does that imply that even older life might have existed among the rocks we can no longer find? How accurate are any of these claims? Scientists (and sometimes religious philosophers) argue that we aren’t even sure about the age of the Earth and the methods we’re using to date these rocks, and the way we date the fossilized lifeforms is usually to assume they’re as old as the rocks we find them in. Valid concerns that samples might have been contaminated by more modern prokaryotes or that isotope ratios might be due to meteorites have been expressed. These candidates for “oldest organism” seem realistic, because their prokaryotic designs are very simple. But without a better understanding of abiogenesis, can we say for sure what the first life would have been like? It may never be possible to answer “why” these are the first organisms, but at least we can try to answer the question “why do we think these were the oldest?” … at least until the next discovery.