Luca is the progenitor of all life on Earth. But its genesis has implications far beyond our planet
A team of scientists based in the UK has revealed some rather surprising news about this well-known ancestor. Although Luca lived a surprisingly long time ago, it was unexpectedly similar to modern bacteria - and what's more, it appears to have lived in a thriving community of other organisms that are now extinct.
The earliest common ancestor of all living things on our planet, known as Luca, is thought to have originated approximately 4.2 billion years ago. At that time, Earth was far from being a paradise. Volcanoes were erupting, and the planet was bombarded by massive meteorites. The world was recovering from a catastrophic event that had caused the Earth's surface to break apart and collide with fragments, eventually giving birth to the moon. This time period, dating before 4 billion years ago, is called the Hadean era in honour of the Greek god of the underworld, Hades.
If Luca really was as ancient as it claims to be, yet still so highly developed and interconnected with its environment, there's an astonishing underlying implication that goes beyond our understanding of human origins. It appears that life on our planet began almost as soon as it feasibly could have. This, in turn, suggests that, when the right conditions and components are present, life might not be an exceptionally unusual and unlikely occurrence, as some scientists have speculated, but rather a natural consequence, and therefore potentially widespread across the universe.
Rika Anderson, microbiologist
Luca's existence is a result of the principles of Darwinian evolution, where all living beings from bacteria to blue whales have descended from earlier forms in one extensive family tree of life. Humans share a common predecessor with chimpanzees and bonobos dating back around 6-8 million years. According to scientists, all monkeys and apes can be traced to a single ancestor around 25 million years ago. The further back you descend this tree, you will find a common ancestor of all mammals, subsequent to that of all vertebrates, and so on.
Luca is the point at which the three life domains – eukarya (encompassing animals, plants and fungi), bacteria, and archaea (a different type of microbe) – eventually merge into a common ancestor. Determining when this occurred has been a long-standing point of contention. Initially, Luca was believed to have existed approximately 3.5-3.8 billion years ago, within a timeframe outside of the Hadean. However, more recent research has proposed that it may be even older than initially thought.
It may seem paradoxical that we can still learn something about Luca, despite the scarcity of contemporary fossil records and the fact that few rocks from that era have remained unchanged. However, scientists can reach conclusions about the characteristics of early organisms using molecular phylogenetics. By comparing the genetic sequences of modern organisms, researchers can determine the order in which various species branched off from a common ancestor based on similarities and differences. They can also infer the genes that their shared ancestors would have had. Noting the rate at which genetic mutations cause such variations provides a "molecular clock" that enables them to estimate when these branching points occurred.
Extending such studies back to the original life on Earth, as far back as Luca, using only genetic information from living organisms today, is a challenging task. The reconstructed genomes of our ancient ancestors are speculative, and incomplete to boot. Hence, the age and characteristics of Luca have been disputed. However, as we continue to gather more genetic information about modern organisms, the accuracy of such reconstructions improves significantly.
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In July last, a team under researchers from the University of Bristol presented a thorough molecular phylogenetic investigation that indicated Luca existed 4.2 billion years ago, give or take 100 million years. This places him at the older end of a range previously suggested by other estimates.
On that ancient Earth, the atmosphere was devoid of breathable air. We know that photosynthesis by plants and bacteria, which only started much later, is what produces oxygen today. Professor Tim Lenton of Exeter University and co-author on the study explains that the atmosphere back then contained a lot of carbon dioxide - suggesting that, "the sky might have had a less vibrant blue hue rather than the one we see today.
The earth at the time was said to have been a predominantly watery environment, covered by oceans with only a few volcanic outcrops breaking through the surface.
Here's the paraphrased text:
How did Luca sustain itself? Molecular analysis reveals it had all the necessary enzymes to obtain sustenance from basic atmospheric gases like carbon dioxide and hydrogen. While based at the sea surface, Luca could have tapped into these resources directly from the atmosphere. Alternatively, the organism may have extracted them from hydrothermal vents deep beneath the ocean, where volcanic heat forces hot water out of rocky formations rich in minerals and gases. Some researchers propose that life first emerged in such underwater vents, shielded from the impact of meteorites.
It would make Luca a chemoautotroph, as that organism is capable of producing the chemicals it needs from simple ones generated by geological processes. Alternatively, it could have relied on chemicals produced through metabolic processes by other organisms within the ecosystem for sustenance. In any case, the research demonstrates that Luca possessed a considerable array of metabolic enzyme machinery, indicating a sufficiently developed and refined system rather than an initial, primitive attempt at life, implying that it had already undergone extensive evolution.
According to Lenton, Luca probably "did not live alone". By producing intricate organic compounds, it would have created a setting where other heterotrophic life forms could flourish, possibly some by feeding on Luca itself. "It would have created ecological niches for other creatures to exploit by utilising its waste products," remarks Philip Donoghue, a palaeobiologist and a leading member of the Bristol research team.
Research conducted by researchers at the University of Arizona in Tucson suggests that this notion is plausible, as they have discovered that proteins were likely first synthesised using sulphur-containing and metal-binding amino acids by the early cell, known as Luca, and its ancient predecessors.
According to the researchers, Luca had an immune system that kept it safe from viral infections. Some bacteria today possess a defence mechanism called CRISPR-Cas, which can 'insert bits of the viral genome into the host DNA', allowing the organism to recall past infections and mount a swift defence, similar to how our own immune systems work. Luca's reconstructed genome has the instructions for a CRISPR-Cas-like system, indicating that viruses were abundant in its environment and were likely an issue for it.
It doesn't come as a shock to some researchers, who think that viruses - organisms that take advantage of host cells' machinery to multiply - are a natural consequence of the DNA replication process. "I tend to view viruses as a universal element of life," says Anderson. However, she notes that she doesn't think ancient viruses resembled those we see today, so she was a bit taken aback to discover that Luca had a CRISPR system. This advanced mechanism is quite sophisticated, especially for an organism that dates back so far.
In the UK English version:
But a world ravaged by viruses wasn't necessarily a bad thing. On the contrary, viruses may have played a key role in creating the thriving ecosystems that once flourished on early Earth. As they can insert new genes into host DNA, viruses can act as messengers for transferring genetic material between organisms through "horizontal gene transfer"; a method by which organisms share genes regardless of their relationship. Luca's ecosystem may have been a breeding ground for virally assisted gene-sharing, resulting in greater diversity than could have emerged through conventional Darwinian evolution through evolution and natural selection.
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It's thought that the earliest ancestry was more of a web-like network, with connections between various forms of life (it still retains some of this structure). The fact that only Luca has continued on to this day might seem like a loss, but horizontal gene transfer could have preserved pieces of this ancient genetic diversity, leaving them within Luca's own DNA.
Anderson suggests that more evidence is needed to confirm the mature age of Luca, and should come from sources such as geological records. Donoghue acknowledges that we shouldn't be certain about Luca's existence, apart from the fact that it did exist. However, this study is unlikely to be the final word.
If it stands, the fact that Luca is so ancient seems to challenge some of the earlier theories for why the universe is mostly devoid of life. "Those were based on the evidence that life took about a billion years to appear on Earth, suggesting that those initial stages were difficult or improbable," notes Donoghue. However, a 4.2bn-year-old and already highly evolved Luca, Lenton explains, "suggests that starting life is not that difficult. It can begin in various places on planets with liquid water, possibly including early Mars or even early Venus."
Studies of stars and planets elsewhere in the universe have found that Earth-like planets aren't as rare as previously thought. However, Anderson notes that it's possible that certain factors unique to our own planet have contributed to its suitability for life.
Besides, says Lenton, the challenge is not just to establish a biosphere but to sustain it: “to have life impact its planetary environment in a manner that maintains it as a habitable world”, as outlined by the Gaia hypothesis proposed by the late scientist and inventor James Lovelock, with whom Lenton collaborated. He holds the viewpoint that Gaia-style maintenance of a biosphere should become a regular occurrence once it has begun. “I am, therefore, predicting that additional biospheres will be found elsewhere in the universe.”
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