If you come to the solar system as soon as it is formed, you will see a completely unfamiliar sight. The mass of our Sun was about the same as it is today, but only a fraction of it, as stars heat up and shine brighter as they age. The four inner rock worlds still exist, but three of them look very similar. Venus, Earth, and Mars all have thin atmospheres, the ability to liquid water on the surface, and the organic components that can produce life. As far as we know, Earth has a large and near moon, Mars has three, and Venus does not. Despite the heat and volcanic activity of all these worlds, they have almost completely abandoned their primordial, hydrogen- and helium-rich shells because light evaporation has boiled them all.
While we still don't know if life ever took root on Venus or Mars, we do know that when the Earth was only a few hundred million years old — probably just 100 million years later, but no later than 700 million years — there was already life on its surface. After billions of years of cosmic evolution that gave rise to the elements, molecules, and conditions that life could have, our planet not only became the place where life appeared, but continued to thrive in the billions of years that followed, even giving rise to us humans, as part of its cosmic story. In terms of our best scientific knowledge, this is what the first steps in life look like.
This tunneling electron microscope image shows some specimens of the cyanobacteria Prochlorococcus marinus. Each of these organisms is only about half a micron in size, but all in all, cyanobacteria are primarily responsible for the Earth's oxygen production: both in the beginning and now. Like all bacteria, they have a much shorter lifespan than humans, and while cyanobacteria are relatively primitive organisms, they "only" date back no earlier than 2.7 billion years, while life on Earth dates back more than a billion years, at least further than that. **Luke Thompson from the Chisholm Lab and Nikki Watson from MIT Whitehead
Life – at least, life as we know it – has some attributes that almost everyone agrees on. While life on Earth involves carbon-based chemistry (which requires carbon, oxygen, nitrogen, hydrogen, and many other elements such as phosphorus, copper, iron, sulfur, etc.) and relies on liquid water, a combination of other elements and molecules may be possible. However, the four general characteristics that are common to all living organisms are as follows:
Life has a metabolism that collects energy from external resources and nutrients for its own use.
Life responds to external stimuli from the environment and changes its behavior accordingly.
Life can grow, adapt to its environment, or it can evolve into a different form from its present form.
Life can reproduce to create viable offspring that arise from its own internal processes.
Although different camps of biologists often argue these points when considering whether something is alive or not (a virus is probably the most hotly contested situation between the two), the current consensus is that all four must be in place at the same time in order to be considered alive.
Snowflakes and crystals may be able to grow and reproduce, but their lack of metabolism prevents them from being classified as live. Proteins may have a metabolism and be able to reproduce, but they do not respond to external stimuli and do not change their behavior depending on the situation they encounter. Even viruses can only multiply by infecting other successful living cells and using them as hosts, which makes one wonder if they are classified as living or non-living.
The animation shows the actual formation and growth of snowflakes, which are ice crystals that can grow into specific configurations, often with hexagonal symmetry. Although crystals have molecular configurations that allow them to reproduce and replicate themselves, they neither utilize metabolic energy nor encode genetic information. While fascinating, snowflakes and all known crystal structures cannot be responsibly defined as "alive". * Vyacheslav Ivanov vimeo
At the same time, no one doubts that the raw materials necessary for the formation of life exist almost everywhere we can see them. Many organic materials – sugars, amino acids, compounds such as ethyl formate, and even complex substances such as polycyclic aromatic hydrocarbons – are found in interstellar space, in asteroids, and were abundant on early Earth. All five nucleobases used in biological processes on Earth were found on asteroids, and perhaps 60 other amino acids, in addition to the 22 amino acids used for biochemical activity on our planet, including amino acids that are chiral opposite to those that play a role in life processes on Earth, were found in meteorites that we subsequently examined in our laboratory.
However, although we have learned about the prebiotic components found in interstellar space, around other stars, and in the original remnant material that remains in our solar system, there is no evidence of any biological activity outside of Earth. Although many have tried to believe that life may have even started in the environment of interstellar space, we don't have any evidence that life began before the formation of Earth.
This conceptual image shows meteoroids transporting all five nucleobases found during life to ancient Earth. All the nucleobases used in life processes, A, C, G, T and U, are now found in meteorites, along with more than 80 amino acids: far more than the 22 known to be used for life processes on Earth. There is no doubt that a similar process has occurred in the star systems of most galaxies throughout the history of the universe. **nasa goddard/ci lab/dan gallagher
Rather, the main idea is that the earth was formed from all these original components, in addition to there may be many more that may later be incorporated into the life process. Perhaps mature nucleotides are common; Perhaps various proteins and protein fragments appeared early and were already pre-assembled; Perhaps the lipid layer and bilayer will emerge spontaneously in the aqueous environment of liquid water. All these possibilities should be considered, but have not been confirmed or proven. The precursors of life, whether they happened to exist in simple or complex states, certainly existed and were abundant in the environment of the early earth.
However, in order to move from life to the precursor of actual life, it is believed that we need the right environment to facilitate the emergence of life from non-life. The three planets that initially created favorable conditions for the emergence of life – Venus, Earth, and Mars – all have:
A reasonable level of surface gravity, an incipient rarefied atmosphere, an environment that allows liquid water to appear on its surface, and all these biochemical precursor molecules.
While all three worlds may have had a chance to form life for the first time, Earth is the only one with substantial evidence that life not only appeared, but appeared relatively early in the history of the solar system.
Volcanic activity on Earth, including the release of large amounts of solid and gaseous substances into our atmosphere, including nitrogen, carbon dioxide, and water, from the earliest times, converts our young hydrogen helium atmosphere into a nitrogen-rich atmosphere CO2 H2O, which will later be further transformed through biological processes. **c. werner et al.， in deep carbon： past and present， cambridge university press， 2019
Soon after the planets formed, the world of these molten surfaces was too hot for liquid water to exist stably on their surfaces because the interiors of these planets took a long geological time to cool. In the early days, volcanic activity was rampant and the surface was very unstable. At the same time, our solar system experienced a period of heavy bombardment in the early days, affecting all celestial bodies: asteroid-shaped and comet-shaped objects hit each of them, leaving scars on their surfaces and creating pathways for underground magma to emerge from below.
However, both of these activities – volcanic activity and impacts – have also brought water to the surface of these planets, and scientists are still debating which mechanism is primarily responsible for transporting most of the water to early Earth (as well as Mars and Venus). Even in the early days, it is likely that the amount of water that existed was enough to create oceans, oceans, lakes, and rivers, but not enough to create a water world completely covered by liquid water. Each of these planets may have continents and oceans, and at the junction of the two, pools of water are formed: areas where water can exist stably on dry land and are subject to various energy gradients. In addition, freshwater melts, including areas with a lot of volcanic activity below, creating stable pools with very low salt content.
This aerial view of Prismatic Springs in Yellowstone National Park is one of the world's most iconic terrestrial hydrothermal features. The color is due to the variety of organisms that live in these extreme conditions and depends on the amount of sunlight that reaches the various parts of the spring. Hydrothermal fields like this are the best candidate sites for life to first appear on young Earth and could be home to abundant life on various exoplanets. Jim Peaco National Park Service.
Consider these situations together. In mineral-rich pools on the surface, where heat and energy gradients are abundant (from geological heat as well as direct sunlight and day-night cycles), molecules and atoms can be combined. Amino acids can be linked to form proteins, dissolved ions can convert these proteins into enzymes, and evaporation cycles can dehydrate many types of molecules, forcing them to join together. Other geochemical phenomena – including precipitation, the presence of minerals, porous fluid flows, and gradients in water activity – may provide opportunities for molecules to come together in novel and interesting ways.
The influence of tides on Earth may be enhanced by the Moon, but even without moons, Venus, Earth, and Mars all have tides from the Sun. However, there is another kind of energy** that Earth possesses that may contribute to the origin of life, which may not be as spectacular on Venus or Mars: thermal activity from the Earth's interior. At the seabed, hydrothermal vents are geological hotspots and excellent candidate sites for the emergence of life. Even today, they are still home to organisms known as extremophiles: bacteria and other life forms that can withstand temperatures that usually break the molecular bonds associated with life processes.
Deep at the bottom of the ocean, around hydrothermal vents, where no sunlight reaches, life on Earth still thrives. How to create life from non-living is one of the biggest unanswered questions in science today, but hydrothermal vents are one of the main sites where the first metabolic processes (precursors of living organisms) may appear first. If life can exist on Earth, perhaps at the bottom of Europa or Enceladus, then there is life there as well. NOAA Office of Ocean Exploration and Research.
These deep-sea vents contain large energy and chemical gradients in which extremely alkaline spout water is mixed with acidic, carbonic acid-rich seawater. Finally, these vents contain sodium and potassium ions, as well as calcium carbonate structures that can serve as potential templates for first or primitive cells. The fact that life exists in such environments suggests that worlds like Europa or Enceladus may be home to life elsewhere in the modern solar system.
But perhaps the most likely place on Earth to start life is the best in the world: hydrothermal fields. Volcanic activity occurs not only under the ocean, but also on land. Below the freshwater areas, these areas of volcanic activity provide additional heat and energy**, stabilizing temperatures and providing energy gradients. All the while, these locations still allow for the evaporation concentration cycle, providing a closed environment that allows the right ingredients to accumulate, while also allowing sunlight to be exposed to the cycle at night.
On Earth, we can be sure that tide pools, hydrothermal vents, and hydrothermal fields are common. While many of the precursor molecules certainly originated outside of Earth and were later brought here, it is likely that it was here, on Earth, where the transformation of non-life to life occurred spontaneously, and that some sort of nucleic acid-peptide coevolution provided the most favorable pathway to date.
If life begins with a random peptide that can metabolize nutrients from its energy, then replication may arise from peptide-nucleic acid coevolution. Here, DNA-peptide coevolution is illustrated, but it can use RNA or even DNA as nucleic acids. Asserting that life requires a "divine spark" is a classic "god of the crevices" argument, but it is also a fallacy to assert that we know exactly how life arises from non-life. **a.Chotera et al., European Chemistry, 2018.
However, the earliest molecules with the property of metabolizing energy from nutrients and reproducing themselves are unlikely to survive and thrive in the long term. Without membranes to collect nutrients and protect their internal workings from the harsh external environment, many of these "metabolic replicators" could become extinct in a short period of time. Another step, perhaps even another leap, may need to be taken before the ability to respond to environmental external stimuli is acquired. Once this ability, coupled with the ability to grow, change, or evolve (which may already be present once nucleic acids are present), join these metabolic replicators, we can determine that moment and say that mature life has truly emerged.
Over time, the Earth has changed dramatically, and so have the creatures on our planet. We don't know if life is once, more than once, or in different places. What we do know, however, is that if we reconstruct the evolutionary tree of every living organism found on Earth today, they all share the same common ancestor. By studying the genomes of living organisms, biologists can reconstruct the time scale of what is known as LUCA: the last universal common ancestor of life on Earth. Less than a billion years after the Earth, life had the ability to transcribe and translate information between DNA, RNA, and proteins, mechanisms that exist in all descendant organisms today. Whether life has appeared more than once is unknown, but it is generally accepted that all life forms that exist today are indeed descendants of a single group.
In the early days shortly after the initial formation of the Earth, life was likely to appear in the waters of our planet. The evidence we have that all life that exists today can be traced back to a universal common ancestor is very strong, but many details about the early stages of our planet, perhaps the first 1 to 1.5 billion years, remain largely obscure. Although life appeared very early, there is no evidence that life existed on Earth, and the origin is uncertain within 100 million to 700 million years after the formation of our planet. **h.Betts et al., Nature Ecology and Evolution, 2018.
Although geological processes tend to obscure the fossil record hundreds of millions of years later, we have been able to trace the origins of life very far away. Microbial fossils have been found in sandstone dating back 3.5 billion years. Graphite is deposited in metasedimentary rocks that can be traced back to biological origins, dating back 3.8 billion years. Until then, very few dates back to this time have been found in the geological record, but we can be fairly certain – based on the most direct evidence available – that life flourished on Earth about 3.8 billion years ago. That's impressive for a planet that formed just 4.5 billion years ago!
In earlier, more extreme times, deposits of certain crystals in rocks may have originated from biological processes (which is more hotly debated), suggesting that the Earth was born as early as 4It was teeming with life between 3 and 4.4 billion years ago: 100 million to 200 million years after the formation of the Earth and the Moon. If these zircon crystals contain inclusions that could indicate that the metamorphic remains of organic matter, really from life processes, then the impact is staggering. This means that even in times of heavy bombardment, life on Earth remains: perhaps almost as long as the Earth itself.
These tiny zircon crystals, which are only as thick as a human hair, are more than 4 billion years old and hold a wealth of chemical information about the early Earth. The silicon, oxygen, trace elements, and isotopic content in these zircons and their parent magma suggest that plate tectonics existed on Earth more than 4 billion years ago. The graphite deposits found in these zircons show significant and interesting carbon isotope ratios, which suggest but are not evidence of biological origin. Smithsonian Institution.
At some point on our planet, at a very early stage, it becomes rich in precursor molecules that, if combined in the right way, have the potential to give rise to life. Under the right environmental and chemical conditions, these molecules are held together in a way that allows them to perform two important tasks at the same time:
metabolize energy, and copy or replicate yourself.
At some point, those early metabolic replicants, perhaps already classifies as life, or may need more to truly be considered "alive", gain the ability to respond to their environment, as well as the ability to grow, adapt, and even evolve. Even if we are unable to identify these primitive life forms today, and even if we are unsure of the exact mechanism by which it occurred, these events do represent the origin of life on Earth. In a string of utterly uninterrupted biological successes, our planet has been a living world ever since.
While Venus and Mars may have similar opportunities, fundamental changes in Venus' atmosphere make it a scorching greenhouse world in just 200 million to 300 million years, while the death of Mars' magnetic field causes its atmosphere to be stripped away, making it solid and frozen after about 1-1.5 billion years. While an asteroid impact may subsequently send life away from Earth, where it may still travel throughout the solar system or even the Milky Way, all the evidence we have suggests that Earth is here, where Earth's biological activity begins.
After the big ** is about 9It took between 4 and 10 billion years for the Earth to go from a barren, lifeless state to a life-rich state. Since then, we've never looked back.