The further back in time you go, the patchier our understanding of life on Earth gets. That's because fossils from those early years are extremely hard to come by and interpret, for a number of reasons. Now, British scientists have used a different method known as a molecular clock to plot out a rough timeline of all life on Earth, tracing the first organisms back to about 4.5 billion years ago.
The fossil record is relatively rich back till around the Cambrian Period, when life exploded in diversity about 540 million years ago. Before then life was much simpler and didn't necessarily have the kinds of hard tissues that fossilize well, so the record largely rests on trace fossils – things like tracks and burrows that indicate the presence of life, rather than their bodily remains.
The fossil record gets murkier towards the Archaean period, over 2.5 billion years ago. The oldest confirmed fossils are about 3.4 billion years old, but other potential evidence includes a 3.5 billion-year-old hot spring in Australia, 3.7 billion-year-old stromatolites in Greenland, and the oldest (but most contentious) could be as ancient as 4.3 billion years.
"There are few fossils from the Archaean and they generally cannot be unambiguously assigned to the lineages we are familiar with, like the blue-green algae or the salt-loving archaebacteria that colors salt-marshes pink all around the world," says Holly Betts, lead author of the new study. "The problem with the early fossil record of life is that it is so limited and difficult to interpret – careful reanalysis of the some of the very oldest fossils has shown them to be crystals, not fossils at all."
While fossils are great as propping up our overall understanding of the timeline of life, they can't paint a complete picture. After all, life had to have developed enough to leave fossils behind in the first place.
To figure out what came before, researchers have another tool in their belt called a "molecular clock." This system is almost like searching for fossils in the genetic code of living organisms. In a very simplified sense, genetic mutations happen at a relatively consistent rate, and by comparing the genomes of two living species, the evolutionary pathways in both creatures can be traced back to their last common ancestor.
Using this technique and cross-referencing it with the fossil record, researchers from the Universities of Bristol and Bath in the UK developed a timescale for when the widest groups of organisms diverged from one another. The study involved molecular data of 29 genes from across 102 living organisms, including bacteria and other single-celled organisms, plants and animals. These were then checked against nine fossils to ensure the timeline remained consistent.
According to the new timescale, eukaryotes (complex organisms like plants and animals) are relative latecomers to the party, appearing roughly 1.8 billion years ago. The two main branches of the tree of life, bacteria and archaea, diverged about 3.5 billion years ago. Tracing it back further, the researchers estimate that the Last Universal Common Ancestor (LUCA) – a hypothetical single cell that all life on Earth is derived from – probably lived around 4.5 billion years ago.
Life wouldn't have been easy in those very early days. LUCA likely lived through a tumultuous period in Earth's history, arising just after a cataclysmic impact with a protoplanet dubbed Theia, which is believed to have spawned the Moon. Between 4.1 and 3.8 billion years ago, Earth was regularly being bombarded by meteors, which wouldn't have made for a particularly habitable planet.
Still, life has proven surprisingly hardy, and will probably stick around until the Sun swallows up our little planet in about 5 billion years time.
Of course, none of this is set in stone. The researchers say that their timeline lays out a general skeleton of evolution, but is still flexible enough to be updated as new evidence arises.
The research was published in the journal Nature Ecology and Evolution.
Sources: University of Bristol, The Conversation
