A question that has long dogged scientists is when did life first emerge on Earth? The answer to this question will provide context and insight to our understanding of how life developed and evolved over time on this planet. This conjures up the iconic scene from Star Trek: The Next Generation, when the Q entity transports Captain Picard to prehistoric Earth, riddled with intense volcanic activity. He points to a slimy, green puddle and says “this is you…right here, life is about to form on this planet for the very first time…the building blocks of what you call life…everything you know, your entire civilization, it all begins right here in this little pond of goo.”
Moreover, it will aid in our ongoing search for life beyond Earth.
To put it into context, Earth formed approximately 4.5 billion years ago, and until recently, it was believed that life emerged on this planet just prior to 3.7 billion years ago. Many estimates for the genesis of life on Earth have been proposed over the years, but as researchers amass new evidence, that number continues to be revised to a date closer and closer to the formation of Earth. Now, new findings are forcing another revision; this time indicating that the point at which life first developed on this planet dates back 100 million years earlier than previously believed.
This is an extraordinary claim since it would put the origin of life on Earth in close proximity to when the Earth first coalesced; a time when the planetary environment has been postulated to be quite hostile to advent of primordial organisms.
Pinning down the precise time that life began on Earth has proven difficult. Just three years ago, a group of chemists suggested that the carbon isotope ratios detected in graphite samples that had been collected from zircon found in Western Australia inferred that the carbon was processed inside living organisms. They based that assertion on the fact that the enzymes found in cellular life that fix inorganic carbon tend to use carbon-12, which is why a high carbon-12 to carbon-13 ratio is frequently cited as an “indicator of life.” The zircon samples were determined to be 4.1 billion years old and the graphite trapped inside was believed to be older than that. Therefore, if the carbon ratios are indicative of life, this finding would force the estimated emergence of life back approximately 300 million years from the time period that was generally accepted in 2015.
Earlier this year, researchers discovered a fossil of what appears to be a fragment of seaweed dated to be from 1.6 billion years ago, which forced scientists to consider that multicellular life might have evolved on Earth about 1 billion years earlier than previously believed. Those earlier estimates were based on recovered fossils that originated 600 million years ago. That revised time period seemed incongruous with evidence suggesting that the oxygen levels at that point in Earth’s history were too low to support the development of complex multicellular lifeforms.
These two studies clearly show that the evidence being amassed pushes the inception of life on Earth back further and further in time. The most recent report continues that trend, suggesting a much earlier date for the origin of the first seeds of life on this planet. Validating these discoveries has been difficult given the fragmented nature of the fossil record and the fact that upon re-examination many of the oldest fossils have been shown to be nothing more than crystals.
A team of scientists dedicated to the search for the “last universal common ancestor” (or LUCA) published results in Nature Ecology and Evolution that point to LUCA’s emergence at a date prior to the “late heavy bombardment” (or LHB) or what is also described by some as the “lunar cataclysm,” which took place between 3.8 to 4.1 billion years ago.
What would constitute fledgling life? Biologists have posited that it would likely be a cluster of microscopic cells that would be the progenitors for the current three, possibly four, existing domains of life. Others have proposed that life may have originated first as a series of pre-cellular replicative molecules. These prototypic molecules, likely the predecessors of nucleotides and amino acids, were distinctive in their ability to undergo template-driven propagation from monomers to polymers (ScienceDaily). Such a model will be difficult to prove given that these chemicals would not be represented in the fossil record.
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.
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