How did life begin on Earth? Although scientists have several theories, they still do not fully understand the precise chemical steps that led to the emergence of biology, nor when the first primitive life forms appeared.
But what if life on Earth had not been born here, having arrived in meteorites from Mars? It’s not the most accepted hypothesis for the origin of life, but it remains an intriguing idea. Here we will look at the evidence for and against.
Time is a fundamental factor. Mars formed around 4.6 billion years ago, while Earth is slightly younger, at 4.54 billion years old. The surfaces of both planets were initially molten, before gradually cooling and hardening.
In theory, life could have appeared independently on Earth and Mars shortly after the planets formed. Although the surface of Mars today is uninhabitable for life as we know it, early Mars likely had conditions similar to early Earth.
Early Mars appears to have had a protective atmosphere and liquid water in the form of oceans, rivers and lakes. It may also have been geothermally active, with numerous hydrothermal and geothermal vents to provide the conditions necessary for life to emerge.
But about 4.51 billion years ago, a rocky planet the size of Mars called Theia collided with proto-Earth. This impact caused the two bodies to merge and then separate, forming the Earth and the Moon. If life had begun before this event, it certainly would not have survived.
Mars, on the other hand, probably did not experience a global overhaul event. The Red Planet suffered several impacts during the violent early days of the solar system, but evidence suggests that none of them would have been large enough to completely destroy the planet – and some areas may have remained relatively stable.
Therefore, if life had appeared on Mars shortly after the planet’s formation 4.6 billion years ago, it could have continued to evolve without major interruption for at least half a billion years. After this period, Mars’ magnetic field collapsed, marking the beginning of the end of Martian habitability. The protective atmosphere is gone, leaving the planet’s surface exposed to the freezing temperatures and ionizing radiation of space.
A question of time
But what about Earth? How long after the impact that formed the Moon did life appear? Following the tree of life to its root, we come to a microorganism called Luca – an acronym for “last universal common ancestor”. This is the microbial species from which all current life descends. A recent study reconstructed Luca’s characteristics using genetics and the fossil record of early life on Earth. He deduced that Luca lived 4.2 billion years ago, earlier than some previous estimates.
Luca was not the first organism on Earth, but one of many species of microbes that existed on our planet at that time. They competed, cooperated, and survived the elements, all while defending themselves against virus attacks.
If small but fairly complex ecosystems existed on Earth around 4.2 billion years ago, life must have arisen before that. But how long before? Luca’s new age estimate is 360 million years after the formation of the Earth and 290 million years after the impact that formed the Moon. All we know is that over the course of 290 million years, chemistry somehow became biology. Was this enough time for life to arise on Earth and then diversify into the ecosystems present during Luca’s lifetime?
The Martian origin of terrestrial life circumvents this problem. According to the hypothesis, Martian microorganisms could have traveled to Earth on meteorites, just in time to take advantage of the favorable conditions for life on the planet after the formation of the Moon.
Perhaps the time has come for this idea. But as someone working in this field, I would guess that 290 million years is enough time for chemical reactions to produce the first living organisms on Earth and for biology to diversify and become more complex.
Survive the journey
Luca’s reconstructed genome suggests it could feed on molecular hydrogen or simple organic molecules. Along with other evidence, this suggests that Luca’s habitat was a shallow marine hydrothermal vent system or geothermal hot spring. Current thinking about the origin of the domain of life is that these types of environments on early Earth presented the conditions necessary for life to arise from non-living chemistry.
Luca also contained a biochemical mechanism that could protect it from high temperatures and UV rays – real dangers in these early terrestrial environments.
However, it is far from certain that primitive life forms could have survived the journey from Mars to Earth. And nothing in Luca’s genome suggests it was particularly well suited to spaceflight.
To reach Earth, the microorganisms would have to have survived the initial impact on the surface of Mars, a high-speed ejection from the Martian atmosphere, and a journey into the vacuum of space while being bombarded by cosmic rays for at least most of the year.
They would then have to have survived a high-temperature entry into the Earth’s atmosphere and another impact on the surface. This latter event may or may not have deposited them in an environment to which they were remotely adapted.
The chances of all this happening seem very low to me. As difficult as the transition from chemistry to biology may seem, it seems much easier to me than the idea that this transition would occur on Mars, with life forms surviving the journey to Earth and then adapting to an entirely new planet. But I could be wrong.
It is useful to review studies investigating whether microorganisms could survive travel between planets. So far, it appears that only the most resilient microorganisms could survive the journey between Mars and Earth. They are species adapted to prevent radiation damage and able to survive desiccation through the formation of spores.
But maybe, just maybe, if a population of microorganisms were trapped inside a large enough meteorite, it could be protected from most of the harsh conditions of space. Some computer simulations even support this idea. Additional simulations and laboratory experiments to test this are underway.
This raises another question: If life arrived on Earth from Mars during the first 500 million years of our solar system’s existence, why didn’t it spread from Earth to the rest of the solar system over the next four billion years? Maybe we’re not Martians after all.
This text was published in The Conversation. Click here to read the original version.
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