The planet has had life on it, in some form or another, for nearly as long as Earth has existed.
If you came to our Solar System right after it formed, you would have seen a completely foreign-looking sight. Our Sun would have been about the same mass it is today, but only about 80% as luminous, as stars heat up as they age. The four inner, rocky worlds would still be there, but three of them would look extremely similar. Venus, Earth, and Mars all had thin atmospheres, liquid water on their surface, and the organic ingredients that could give rise to life.
While we still don’t know whether life ever took hold on Venus or Mars, we know that by the time Earth was only 100 million years old, there were organisms living on its surface. After billions of years of cosmic evolution gave rise to the elements, molecules, and conditions from which life could exist, our planet became the one where it not only did, but where it thrived. To the best of our scientific knowledge, here’s what those first steps were like.
Life as we know it has a few properties that everyone agrees on. While life on Earth involves carbon-based chemistry (requiring carbon, oxygen, nitrogen, hydrogen, and many other elements like phosphorous, copper, iron, sulfur, and so on) and relies on liquid water, other combinations of elements and molecules may be possible. The four general properties that all life shares, however, are as follows:
- Life has a metabolism, where it harvests energy/resources from an external source for its own use.
- Life responds to external stimuli from its environment, and alters its behavior accordingly.
- Life can grow, adapt to its environment, or can otherwise evolve from its present form into a different one.
- And life can reproduce, creating viable offspring that arise from its own internal processes.
All four of these must be in place, simultaneously, for a population of organisms to be considered alive. Snowflakes and crystals may be able to grow and reproduce, but their lack of a metabolism prevents them from being classified as alive. Proteins may have a metabolism and be able to reproduce, but they do not respond to external stimuli or alter behavior based on what they encounter. Even viruses, which are the most debatable organism on the line between life and non-life, can only reproduce by infecting other successfully living cells, casting doubt on whether they’re classified as living or non-living.
Many organic materials — chemical compounds like sugars, amino acids, ethyl formate, and even complex ones like polycyclic aromatic hydrocarbons — are found in interstellar space, in asteroids, and were abundant on early Earth. But we do not have evidence that life began prior to Earth’s formation.
Instead, the leading thought is that the Earth was formed with these raw ingredients on it, and perhaps many more. Perhaps nucleotides were common; perhaps proteins and protein fragments came pre-assembled; perhaps lipid layers and bilayers could spontaneously arise in an aqueous environment. In order to go from precursors to life to actual life, however, it’s believed that we needed the right environment.
These three favorable planets — Venus, Earth, and Mars — all likely had a reasonable level of surface gravity, thin atmospheres, liquid water on their surfaces, and these biochemical precursor molecules. The one thing Earth had that the other two planets likely didn’t, however, was a Moon. While all three worlds likely had a chance to form life for the first time, our Moon helped give us chances that the other worlds may not have had.
The amount of water present on these early planets was very likely enough to create oceans, seas, lakes, and rivers, but not enough to completely cover them in liquid water. This means they all had continents and oceans, and at the interface of the two, there were tidepools: regions where water can stably exist on dry land and be subject to all sorts of energy gradients.
Sunlight, shadow and night, cycles of evaporation and concentration, porous fluid flow in the presence of minerals, and gradients of water activity could all provide opportunities for molecules to bind together in novel and interesting ways. The effects of tides may be enhanced by the Moon, but all these worlds possess tides due to the Sun. However, there’s an additional energy source that the Earth possesses that likely contributed to life’s origin, that may not have been as spectacular on Venus or Mars.
That latter factor is thermal activity from the interior of the planet. At the bottom of the oceans, hydrothermal vents are geological hotspots that are excellent candidate locations for life to arise. Even today, they are home to organisms known as extremophiles: bacteria and other lifeforms that can withstand the temperatures that typically break the molecular bonds associated with life processes.
These vents contain enormous energy gradients as well as chemical gradients, where extremely alkaline vent water mixes with the acidic, carbonic-acid-rich ocean water. Finally, these vents contain both sodium and potassium ions, as well as calcium carbonate structures that could serve as a template for the first cells. The fact that life exists in environments like this points to worlds like Europa or Enceladus as potential homes for life elsewhere in the Solar System today.
But perhaps the most likely location for life to begin on Earth is the best of all worlds: hydrothermal fields. Volcanic activity doesn’t solely occur beneath the oceans, but also on land. Beneath areas of fresh water, these volcanically-active areas provide an additional heat and energy source that can stabilize temperatures and provide an energy gradient. All the while, these locations still allow evaporation/concentration cycles, provide a confined environment that enables the right ingredients to accumulate, and allow a sunlight/night cycle of exposure.
On Earth, we can be confident that tidepools, hydrothermal vents, and hydrothermal fields were all common. While the precursor molecules certainly originated beyond Earth, it was likely here on our planet that the transformation of non-life into life spontaneously occurred.
Over time, the Earth has changed tremendously, as have the living organisms on our planet. We do not know if life arose once, more than once, or in disparate locations. What we do know, however, is that if we reconstruct the evolutionary tree of every extant organism found on Earth today, they all share the same ancestor.
By studying the genomes of the extant organisms found on our world today, biologists can reconstruct the timescale of what’s known as LUCA: the Last Universal Common Ancestor of life on Earth. By time the Earth was less than 1 billion years old, life already had the ability to transcribe and translate information between DNA, RNA, and proteins, and these mechanisms exist in all organisms today. Whether life arose multiple times is unknown, but it is generally accepted that life as we know it today descended from a single population.
Despite the fact that geological processes can often obscure the fossil record beyond a few hundred million years, we have been able to trace back the origin of life extraordinarily far. Microbial fossils have been found in sandstone dating to 3.5 billion years ago. Graphite, found deposited in metamorphosed sedimentary rock, has been traced back to having biogenic origins, and dates back to 3.8 billion years ago.
At even earlier, more extreme times, the deposits of certain crystals in rocks appear to originate from biological processes, suggesting that Earth was teeming with life as early as 4.3 to 4.4 billion years ago: as soon as 100–200 million years after the Earth and Moon formed. To the best of our knowledge, life on Earth has existed almost as long as Earth itself has.
At some point on our planet, in the very early stages, the molecules that are abundant and precursors to life, under the right energy and chemical conditions, began to simultaneously metabolize energy, respond to the environment, grow, adapt, evolve, and reproduce. Even if it would be unrecognizable to us today, that marks the origin of life. In a radically unbroken string of biological success, our planet has been a living world ever since.
While Venus and Mars may have had similar chances, radical changes to Venus’ atmosphere rendered it a searing hothouse world after just 200–300 million years, while the death of the Martian magnetic field caused its atmosphere to be stripped away, rendering it solid and frozen. While asteroid strikes may send Earth-based life off-world, throughout the Solar System and galaxy, all the evidence suggests that we are where it started.
By 9.4 billion years after the Big Bang, Earth was teeming with life. We’ve never looked back.
Further reading on what the Universe was like when:
- What was it like when the Universe was inflating?
- What was it like when the Big Bang first began?
- What was it like when the Universe was at its hottest?
- What was it like when the Universe first created more matter than antimatter?
- What was it like when the Higgs gave mass to the Universe?
- What was it like when we first made protons and neutrons?
- What was it like when we lost the last of our antimatter?
- What was it like when the Universe made its first elements?
- What was it like when the Universe first made atoms?
- What was it like when there were no stars in the Universe?
- What was it like when the first stars began illuminating the Universe?
- What was it like when the first stars died?
- What was it like when the Universe made its second generation of stars?
- What was it like when the Universe made the very first galaxies?
- What was it like when starlight first broke through the Universe’s neutral atoms?
- What was it like when the first supermassive black holes formed?
- What was it like when life in the Universe first became possible?
- What was it like when galaxies formed the greatest numbers of stars?
- What was it like when the first habitable planets formed?
- What was it like when the cosmic web took shape?
- What was it like when the Milky Way took shape?
- What was it like when dark energy first took over the Universe?
- What was it like when our Solar System first formed?
- What was it like when planet Earth took shape?