How Did Life on Earth Begin?

Nobody knows exactly how life on Earth began. What we do know is striking. Earth formed about 4.54 billion years ago, according to the U.S. Geological Survey, and life seems to have appeared fairly soon after. There is good fossil evidence from about 3.5 billion years ago, and possibly earlier. Lab experiments show that some of life’s chemical building blocks can form naturally. But no one has yet turned non-living chemicals into a living cell, and scientists still debate where and how it first happened.

The study of life arising from non-life is called abiogenesis. It is a bit like working out how human language began, using only the languages spoken today and a few ancient inscriptions. The clues are real, but the first words were never recorded.

How early did life appear on Earth?

Some of the oldest widely accepted signs of life are stromatolites: layered rock mounds built up by mats of microbes. In the Pilbara region of Western Australia, researchers from UNSW Sydney reported stromatolites and other microbial traces in 3.48-billion-year-old hot-spring deposits. The layered rock pictured above comes from the same rock formation.

Older claims exist, reaching back 3.7 billion years or more. They are debated. A 2018 review in the journal Astrobiology notes that almost all rocks older than 3.6 billion years were changed by intense heat and pressure, so fossils that old are rarely preserved. The same review estimates that, once Earth became habitable, life took as little as about 200 million years or as long as 800 million years to appear.

Genes tell a similar story. Everything alive today descends from a shared ancestor, nicknamed LUCA (the last universal common ancestor). In 2024, a team led by the University of Bristol compared the genes of living species and estimated that LUCA lived about 4.2 billion years ago. The team said it did not expect LUCA to be so old. They also found it was already a fairly complex cell, not unlike today’s bacteria and other simple cells. If so, life’s very first steps came even earlier. For how life branched out afterward, see How Does Evolution Work?

What have laboratory experiments shown?

Making building blocks

In 1953, chemist Stanley Miller published what became known as the Miller–Urey experiment. He sent electric sparks, standing in for lightning, through a sealed apparatus holding water, methane, ammonia and hydrogen. Amino acids, the building blocks of proteins, formed. Decades later, scientists reanalyzed vials Miller had saved from a “volcanic” version of his experiment. He had identified five amino acids; modern instruments found 22, according to a 2008 study in Science.

That study also names a catch. Geoscientists today doubt that early Earth’s air was as rich in these gases as Miller assumed. The authors suggest that local spots, such as lightning-filled volcanic plumes, could still have worked.

Later work went further. In 2009, chemists at the University of Manchester reported a plausible early-Earth route to two of the four chemical “letters” of RNA, a close cousin of DNA. Until then, no believable natural path to them had been found.

The “RNA world”

Living cells face a chicken-and-egg problem. DNA holds the instructions for building proteins, but cells need protein enzymes to use those instructions. Which came first? In the early 1980s, Sidney Altman and Thomas Cech discovered that RNA can do both jobs: store information and speed up chemical reactions, like an enzyme. That work won the 1989 Nobel Prize in Chemistry, and the prize announcement noted that it might solve the chicken-and-egg problem. That is the heart of the RNA world idea: early life may have run on RNA alone.

In 2009, Tracey Lincoln and Gerald Joyce built pairs of RNA molecules that copy each other when given ready-made RNA pieces, and that evolve in a test tube. As NASA describes it, the system technically meets NASA’s working definition of life: a self-sustaining chemical system capable of Darwinian evolution. Joyce himself cautions that it is not truly alive, because it carries too little information. The RNA world is well supported, but still debated.

Did life start in deep-sea vents or warm little ponds?

This is one of the liveliest debates in the field.

Deep-sea vents. In 2000, scientists found the Lost City vent field on the floor of the Atlantic Ocean. Its white carbonate towers, the tallest about 18 stories high, release strongly alkaline fluids rich in hydrogen and methane, and simple carbon compounds form there without any help from living things. To supporters, vents like these could have supplied the energy and raw chemistry life needed to get started.

Warm little ponds. In an 1871 letter, Charles Darwin imagined a “warm little pond” with ammonia and phosphoric salts, light, heat and electricity, where a protein compound might form. He called it a big “if.” The 3.48-billion-year-old Australian hot-spring deposits led their discoverers to suggest that life may have begun in freshwater hot springs on land, rather than in the ocean.

Both camps have real strengths and real problems. The question is still open.

What don’t we know yet?

• The missing middle. We can make some building blocks, and we can make RNA molecules that copy one another with help. The long road from a chemical mix to a cell that grows, divides and evolves on its own has not been rebuilt in any lab.
• What came first. As NASA’s article puts it, researchers still don’t know which pieces of the first life came first and which came later.
• Where. Deep-sea vents, hot springs on land, or somewhere else entirely.
• Help from space. Material from space reached early Earth, but scientists don’t know how much it mattered for life.

What would change the answer?

• A full recipe in the lab. A step-by-step path from simple chemicals to a self-copying, evolving cell, under early-Earth conditions, would be a landmark.
• Clearer ancient fossils. Less disputed traces in older rocks would narrow down when life began.
• A second origin. Finding life that arose on its own elsewhere, such as on Mars or an icy moon, would suggest life starts readily when conditions allow. We explore that search in Are We Alone in the Universe?

Is this a question for science, faith, or both?

Science looks for natural causes. Philosophers call this methodological naturalism: a working rule that limits scientific research to natural things and laws. As the Stanford Encyclopedia of Philosophy notes, this rule makes no claim either way about whether anything supernatural exists. So science can ask how chemistry became biology. Whether life has an ultimate purpose is a different kind of question, one for philosophy and belief.

A thoughtful believer’s perspective: Natural processes, if they are found, describe how creation unfolded, not whether it had a Creator. The more elegant the chemistry turns out to be, the more reason for wonder and gratitude.

A thoughtful skeptic’s perspective: Many things once explained by divine action, such as lightning or disease, later found natural explanations. So it seems wiser to say “we don’t know yet” than to fill today’s gaps with a Creator.

Both are perspectives, not evidence. Science can inform them, but it cannot settle them. We look more closely in Science and Faith.

The first living thing left no diary. What it left instead was us, and every other living thing, carrying chemical clues in our cells. Darwin called his own guess a big “if.” That small word still fits, and it is a good word to keep asking with.