Abiogenesis

Abiogenesis, also called the origin of life, is the natural way that life begins from things that aren’t alive, like simple chemicals. Scientists think this didn’t happen all at once. Instead, it was a step-by-step process. First, Earth had to be a place where life could survive. Then, simple chemicals came together to form more complex ones. These chemicals eventually learned how to copy themselves and build themselves into tiny structures that could become cells.

Studying abiogenesis helps us understand how the chemicals that made up early life turned into the living things we see today. It mixes ideas from biology and chemistry to look at how life works. Life uses carbon and water, and it depends on special groups of chemicals like lipids for cell walls, carbohydrates for energy, amino acids for building proteins, and DNA and RNA for passing on traits.

One famous experiment in 1952 showed that important building blocks of life could be made from simple chemicals found on early Earth. Since then, we’ve found these building blocks in space, on meteorites, and in comets. Even though we haven’t seen life begin in a lab, many ideas about how it might have happened have been tested. Scientists also study the last common ancestor of all living things today to learn clues about what the first life might have been like.

While Earth is the only place we know where life exists, many scientists think life could have started in similar ways on other planets too. The oldest signs of life on Earth are very old, showing that life began relatively quickly after Earth formed.

Overview

Further information: Astrobiology

Life means being able to make copies of itself with small changes. NASA describes life as a chemical system that can change over time in a special way. This is a very complex idea. The simplest known life form, which lived about 4 billion years ago, already had many pieces of information stored in its DNA and tools to use that information.

Scientists try to understand how such a complicated system could come together step by step. One idea is that simple molecules could start making copies of themselves, and then group together to form more complex structures. We have not seen this change from non-living to living happen in a lab yet.

For life to begin, a planet needs the right conditions, like having water and minerals, and energy from sources such as the Sun. Simple building blocks can form under these conditions, and they can link together into longer chains. While we know how life changed after the simplest cell appeared, we still do not fully understand how those first cells formed from simpler parts.

Although we only know of life on Earth, scientists study other planets to see if life might exist elsewhere. Research focuses on understanding the steps, energy sources, and conditions that helped life begin.

Pre-1960s conceptual history

Main article: History of research into the origin of life

Spontaneous generation

Main article: Spontaneous generation

An old idea from Aristotle until the 1800s was that life could just appear from decaying stuff, like insects coming from rotting meat. But scientists like Robert Hooke and Antonie van Leeuwenhoek started studying tiny living things with microscopes and found that life didn’t just pop up like that. By the middle of the 1800s, people knew this idea wasn’t true anymore.

Panspermia

Main article: Panspermia

Some people, like Anaxagoras from ancient times, thought life didn’t start on Earth but came from somewhere else in space. Today, some scientists think life might have hitchhiked to Earth on rocks from space, like from Mars. This means life could have started on one planet and then traveled to others.

"A warm little pond": primordial soup

Main articles: Primordial soup and Warm little pond

In the 1800s, scientists like Charles Darwin wondered if life began in a cozy pond filled with simple chemicals. Later, in the 1920s, Alexander Oparin and J. B. S. Haldane suggested that the early oceans were like a soup where tiny building blocks of life slowly came together.

Miller–Urey experiment

Main article: Miller–Urey experiment

In 1952, Stanley Miller and Harold Urey did an experiment to see if they could make the building blocks of life from simple chemicals, like what might have been on the very early Earth. They used a mix of gases and found they could create some of these important molecules. Scientists today are still studying how life might have started in places like space or deep underwater.

Producing a habitable Earth

Evolutionary history

Early universe with first stars

Emergence of Earth

Earth is about 4.54 billion years old, as shown by studying very old rocks. When Earth was young, during a time called the Hadean, it was not friendly for life. After Earth formed, it collided with a body called Theia, which created our Moon. This collision changed Earth’s early atmosphere.

Earth’s early atmosphere was made of water vapor, nitrogen, and carbon dioxide, with smaller amounts of other gases. These ingredients may have helped create the oceans. Scientists think the oceans formed very early, perhaps as soon as the Moon was created.

Modern stromatolites in Shark Bay, created by photosynthetic cyanobacteria

Earliest evidence of life

Main article: Earliest known life forms

From studying very old rocks, scientists think life first appeared on Earth between 4.32 and 3.48 billion years ago. Some of the oldest signs of life are tiny structures found in rocks from Quebec and Australia. These structures look like modern microbes that live near deep-sea vents. Other signs of early life include special kinds of carbon found in rocks from Greenland and Australia.

The Pilbara region in Western Australia has very old rocks with layered structures called stromatolites. These layers are made by tiny organisms, similar to ones living today. These findings show that life on Earth began very early, not long after the oceans formed.

Producing molecules: prebiotic synthesis

Further information: Primordial soup and Nucleosynthesis

Almost all the building blocks of life come from simple chemicals that formed in stars, except for hydrogen and a little helium and lithium. The basic parts needed for life, like simple carbon and hydrogen groups, can be made by the bright light from stars. More complicated chemicals, including those needed for life, form naturally in space and on planets. These chemicals could have come from Earth itself, made by energy from things like sunlight, lightning, or impacts, or they might have arrived from space, forming in clouds between stars or in the area around our Sun before landing on our planet.

Observed extraterrestrial organic molecules

Chemicals that contain carbon, which are important for life, are common in the Sun, stars, comets, and the air around most planets in our solar system. These carbon-based chemicals are found often in space, made in big clouds of gas and dust around stars, and they change after being hit by energy from space rays. Important pieces that help make DNA and RNA, like guanine, adenine, cytosine, uracil, and thymine, as well as sugars, have been found in rocks from space that fall to Earth, called meteorites. The amino acid glycine was found in material from the comet Wild 2. Comets are covered with dark, tar-like material made from simple carbon chemicals changed by energy from space. Rain from comets may have brought these complex chemicals to Earth. During a busy time long ago when lots of space rocks hit Earth, meteorites might have brought millions of tons of important chemicals to our planet each year. Today, about 40,000 tons of space dust falls to Earth every year.

Polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAH) are very common chemicals made of carbon that exist everywhere in the universe, and they were likely part of the oceans of early Earth. These chemicals have been found in clouds of gas and dust in space, in comets, and in meteorites.

A young star, HH 46-IR, similar to our Sun when it was young, is surrounded by material that contains molecules such as cyanide groups, hydrocarbons, and carbon monoxide. PAHs in space can change through adding hydrogen, oxygen, and other things to become more complex chemicals used in living cells.

Nucleobases and nucleotides

Further information: Nucleobase and Nucleotide

Organic chemicals brought to Earth by tiny particles from space can help make more complex molecules because of special surfaces that help them stick together. Studies of meteorites suggest that some important pieces for RNA, like uracil, were likely made in space, shown by special types of carbon found in these rocks. Research by NASA suggests that all the basic pieces for DNA were made in space. Space dust contains complex organic materials that could form quickly around stars. Glycolaldehyde, a sugar that helps make RNA, has been found in space around young stars and on meteorites.

Laboratory synthesis

As far back as the 1800s, experiments showed that important chemicals for life can be made from simple carbon materials mixed with common inorganic helpers. Making big molecules from simple building blocks under early Earth conditions is not easy. Along with the needed simple pieces, other chemicals that stop big molecules from forming were also made in high amounts in early experiments. Life uses only 20 special kinds of building blocks for its proteins, which is a tiny part of what could exist. We need to explain why life uses only these few.

Formamide, a chemical made from water and a gas called hydrogen cyanide, might have been a good place where simple materials turned into parts of amino acids.

Sugars

Further information: Formose reaction

The Breslow catalytic cycle for formaldehyde dimerization and C2-C6 sugar formation

In 1861, a scientist showed that when you heat a simple chemical called formaldehyde under certain conditions, it makes sugars like those with four, five, and six carbon pieces. Another scientist suggested in 1959 that this reaction could happen on its own.

Nucleobases

Nucleobases, like guanine and adenine, can be made from simple carbon and nitrogen materials, such as a gas called hydrogen cyanide (HCN) and ammonia. On early Earth, HCN might have been made by sunlight after big impacts. Formamide, made from water and HCN, can make all four pieces needed for RNA when heated with Earth minerals. HCN can also help make the amino acid glycine.

Parts of DNA and RNA, like uracil, cytosine, and thymine, can be made in space using starting materials such as pyrimidine from meteorites. Pyrimidine might have formed around big red stars, in dust between stars, or on Earth after impacts from space rocks. All four RNA building blocks might be made from formamide during big energy events like impacts from space. Scientists have made several RNA pieces in labs that copy conditions from before life existed, using a special reaction called the autocatalytic formose reaction.

Other ways to make these base pieces from simple materials have been found. Cold temperatures help make purines by gathering important materials like HCN. Adenine and guanine need cold, but cytosine and uracil might need hot water. When ammonia and a gas called cyanide were left in a freezer for 25 years, seven amino acids and eleven types of nucleobases formed in ice. Certain chemicals can be made by freezing and heating a solution of urea with sparks, and these reactions happen fast because of how ice holds things together.

Peptides

Prebiotic peptide synthesis could have happened in different ways. Some need very hot and crowded places where joining together is easier, while others use special chemicals that help join pieces. Evidence shows that repeated wet and dry times and certain salts can help simple pieces called glycine join into longer chains. Other work shows that surfaces of certain minerals can help pieces join, but they can also break them apart again. So, more help might be needed to make enough peptides.

Many chemicals that could help join pieces together have been found, like cyanamide, dicyanamide, dicyandiamide, diaminomaleonitrile, urea, trimetaphosphate, NaCl, CuCl_2, (Ni,Fe)S, CO, carbonyl sulfide (COS), carbon disulfide (CS₂)_, SO_2, and diammonium phosphate (DAP).

A 2024 experiment used a special hot, cracked surface to gather important building blocks from a weak mix, which could help make big molecules like peptides. Clays might also help, but this idea hasn’t been proven in experiments.

Making peptides directly from simple gases like CO, NH₃ and C, without first making amino acids, is also very efficient.

Producing protocells

Main article: Protocell

Further information: Gard model, Self-organization § Biology, and Cellularization

Scientists wonder how very simple early cells, called protocells, first appeared and began to change over time. One idea is that the first self-replicating objects were made of special fats called lipids. These lipids can form layers in water, creating tiny bubbles. These bubbles might have protected important molecules and allowed them to copy themselves. Over time, some bubbles would have been better at copying and surviving than others.

A protocell is thought to be a step toward life, made of organized fats that can change and evolve. Scientists have not yet made a protocell that can evolve like real cells, but they are studying how these early cells might have worked. They believe that competition for materials could have helped some protocells become better at surviving and copying themselves. This process might have led to the first simple living cells.

Producing biology

Life needs a way to organize molecules, which seems opposite to nature’s usual tendency toward disorder. But this organization doesn’t break nature’s rules. Living things create order in their own bodies while producing disorder elsewhere, like as heat or waste.

Early Earth had many energy sources that could help simple chemicals become more complex. Heat from Earth’s interior, sunlight, lightning, and even falling space rocks could all provide energy. Experiments show these energies can help make important building blocks for life. Special surfaces made of iron and sulfur may have helped create early molecules like amino acids.

Suitable geological environments

Many ideas exist about where life might have first started on Earth. These ideas often differ because scientists think about different things like what kinds of materials were available, what the Earth was like long ago, and what early life might have needed.

The earliest known life forms may be putative fossilized microorganisms, found in white smoker hydrothermal vent precipitates. They may have lived as early as 4.28 Gya (billion years ago), relatively soon after the formation of the oceans 4.41 Gya, not long after the formation of the Earth 4.54 Gya.

Deep sea hydrothermal vents

One idea is that life began near deep sea hydrothermal vents. These are places in the ocean floor where hot, chemical-rich water comes out. Some scientists think that the special conditions here could have helped simple molecules come together to form the first living things. These vents have minerals that might have helped chemical reactions happen more easily.

Surface bodies of water

Another idea is that life started in surface water like lakes, rivers, or hot springs. These places can go through wet and dry times, which might help simple molecules become more complex. Some scientists think that places like hot springs, with their mix of water, minerals, and heat, could have been good for early life to form.

Inside the continental crust

Some scientists think life might have started deep inside the Earth’s crust, in cracks and faults. These places could have been protected and had the right mix of water, chemicals, and energy to help life begin. The changing conditions inside these cracks might have helped simple molecules become more complex over time.

Homochirality

Main article: Homochirality

Many biomolecules, such as L-glutamic acid, are asymmetric, and occur in living systems in only one of the two possible forms, in the case of amino acids the left-handed form. Prebiotic chemistry would produce both forms, creating a puzzle for abiogenesis researchers.

Homochirality is the idea that living things use molecules that all look the same in one special way. For example, the building blocks of proteins, called amino acids, all twist one way, while the parts that make up DNA and sugars twist the opposite way. Usually, when these molecules are made, they come out in equal amounts of both twists. But under certain conditions, like exposure to special light, more of one twist can appear. Some scientists think this might have happened when materials from space reached Earth a long time ago. Once one twist becomes more common, it can influence how more complex molecules form.