The fossil record is frustratingly incomplete and the earliest fossils date back approximately 3.8 billion years. Philip Donoghue, one of the co-authors of the published study, describes a “second record of life” that can be used to supplement the fossil record, which exists inside the genomes of all living organisms found on modern Earth. Here, the research that allowed scientists to revise their estimate of the date of life’s emergence on Earth relied upon the use of so-called “molecular clocks.” A molecular clock is established by counting the number of mutations in the genomic DNA of several species with the supposition that this number is proportional to how far back in time their lineages diverged from a common ancestor.
To that end, the research team behind this recent study combed the genetic material of 102 separate organisms for changes in the DNA code of 29 distinct genes. The data generated from this exhaustive search allowed them to develop a timeline from which all significant clusters of life originated on Earth from bacteria to advanced multi-cellular organisms. As stated earlier, the “molecular clocks” have inferred that LUCA sprung into existence far earlier than the existing fossil record suggested; right before the late heavy bombardment.
The idea that life originated during this cataclysmic period was unexpected and remains controversial to say the least. This would place LUCA’s emergence just after a major event in the geological history of Earth occurred: when another planet named Theia collided with the early Earth, breaking off large chunks that would coalesce into the moon. Such a collision would have certainly destroyed any life that may have been present at that time. And many scientists believe that the environmental conditions on Earth after that impact would have been far too volatile to have supported the emergence of life.
It is highly unlikely that any fossil record of LUCA will be found given that the earliest life would consist of microscopic cells, the artifacts of which are difficult to identify and validate. Therefore, any “molecular clock” evidence will lack corroboration from the fossil record and likely to remain a subject of much contention. Barring Q’s intervention, the precise date that life first arose on Earth will continue to fuel much debate in the coming years.