“That Is a Lot of Evolution”: Study Finds LUCA Required 2,600 Genes
Recently I wrote about a study published in Nature Ecology & Evolution which found that the origin of life on Earth “required a surprisingly short interval of geologic time.” But I didn’t mention that the study reported the astounding complexity they inferred must have been present in that life — namely the Last Universal Common Ancestor (LUCA) of all living organisms. In short, they believe that LUCA must have had some 2,600 protein-coding genes — not that much different from many free-living bacteria or archaea that are around today. From the technical paper:
Phylogenetic reconciliation suggests that LUCA had a genome of at least 2.5 Mb (2.49–2.99 Mb), encoding around 2,600 proteins, comparable to modern prokaryotes. … Altogether, our metabolic reconstructions suggest that LUCA was a relatively complex organism, similar to extant Archaea and Bacteria.
“A Fairly Large Genome”
Similarly, a write-up in Science expands on the meaning of the complexity:
The last ancestor shared by all living organisms was a microbe that lived 4.2 billion years ago, had a fairly large genome encoding some 2600 proteins, enjoyed a diet of hydrogen gas and carbon dioxide, and harbored a rudimentary immune system for fighting off viral invaders.
Likewise a commentary in Nature Ecology & Evolution states
Our results indicate that the LUCA existed between 4.09 and 4.33 billion years ago, a few hundred million years after the moon-forming impact. Our reconstruction of the genome of the LUCA is over 2.5 megabases, comparable to living bacteria, and encompasses at least 2,500 protein-coding genes. The LUCA was capable of nucleotide and protein synthesis, possessed a cellular envelope, and used ATP as an energy currency. … We also found that the LUCA possessed an RNA-based immune system … LUCA must have been part of a broader ecosystem, of which it represents the only living descendant.
Although some aspects of our study are in good agreement with previous work on the LUCA, we infer a larger genome size and genetic repertoire than most previous studies.
This means that not only did life emerge rapidly, but life with a genome composed of thousands of genes and with millions of base pairs emerged rapidly. This is astounding.
Evolutionary Reasoning at Work
Now we must understand that their inferences about LUCA are all based upon evolutionary reasoning. LUCA of course is a hypothetical evolutionary postulate. As such, it depends on universal common ancestry being true. I’m a skeptic of universal common ancestry for various scientific reasons. However, once you understand the reasoning they use to infer the complexity of LUCA, you appreciate the conundrum that this kind of a study poses for naturalistic accounts of the origin of life. An article in the Washington Post invites readers to “Meet the surprisingly complex ancestor of all life on Earth.” It explains the methods that were used:
The new timeline and details can be chalked up to more advanced analysis methods available today. In the new study, the team of 19 scientists used a combination of genetic analysis and fossil records to determine the age of LUCA and its characteristics. They first compared genes in modern genomes of bacteria and archaea to determine which gene families were present in LUCA. They estimated LUCA’s genome size, the number of proteins it encoded and its metabolism.
In other words, by comparing the genomes of modern organisms, they were able to infer the minimal sets of genes that must have been present if in fact they are all descended from a common ancestor. The bottom line? It’s a lot of genes — making LUCA more complex than many evolutionary theorists probably expected to find. As the Washington Post noted:
In the most extensive analysis of the organism to date, scientists propose in a new study that this hypothesized ancestor was more sophisticated than previously known — thought to possess an immune system to fight off viruses, for instance.
The team said LUCA appeared around 4.2 billion years ago, shortly after Earth was thought to be habitable, suggesting it evolved even quicker than previous estimates and survived through tumultuous times on the planet.
[…]But LUCA may have been more complex than they previously thought, the authors found. They inferred it had an immune system that fought viruses and found evidence suggesting it contained genes to protect against ultraviolet damage and lived at the ocean surface.
The lead author of the study was quoted as saying, “[T]his was a fairly complex organism, already possibly by the time of like 4.2 billion years ago.” Another scientist commented, “LUCA was a very complex cell, with a genome similar to modern bacteria (which we think of as simple, but from a molecular biology perspective are very complex).”
That latter scientist was further quoted saying: “That is a lot of evolution to happen within 100 million years or less.” A lot of evolution, indeed.
An Unanswered Question
This leaves unaddressed the question of whether organismal life, a “very complex cell,” could exist at all below the threshold of complexity that the study attributes to LUCA — without, for example, sophisticated defenses such as an “immune system that fought viruses” or “genes to protect against ultraviolet damage.”
Would a much simpler cell be viable? To think so pushes the envelope of plausibility. If it would be viable, someone needs to explain how such a defenseless life could survive to reproduce. If it wouldn’t, the strong suggestion is of life springing from non-life without a far simpler yet viable transitional state.
Transitions like that, including very rapid ones, are a hallmark of human-devised technology. In the context of the early Earth, it sounds like the act of a creative agent, existing before the first cell came to be. In other words, it sounds like intelligent design.
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