In every cell of your body, there are mysterious speckles. You need a microscope to see them, but if you peer closely, you will see lots of tiny dots: some sitting still, some moving around as if swept along in invisible currents.

They look solid, but are actually liquid, and although they were discovered only in 2009, we now know they perform a host of crucial jobs. If they go wrong, they can lead to disaster – in fact, these blobs malfunctioning in the brain may even be a cause of Alzheimer’s disease. Minuscule though they are, they are essential to our survival.

They may also help explain one of the biggest mysteries in biology. Over the past decade, experiments have shown that these droplets may have been crucial to the origins of life. If that’s true, then swimming around in our cells are relics of the first life on Earth.

Depending on whom you talk to, the tiny dots are either called coacervates (pronounced “coh-AH-ser-vates”) or condensates (or sometimes biomolecular condensates). These terms don’t quite map onto each other one-to-one, but the differences are subtle and depend on context. If you are studying them in a test tube, they are typically known as coacervates; if they are in a cell, we call them condensates. “Fundamentally, they’re the same,” says Evan Spruijt, a biophysicist at Radboud University in Nijmegen, the Netherlands.

In the late 19th and early 20th centuries, there was a lot of interest in materials that blurred the lines between solids and liquids, such as liquid crystals and gels. Coacervates were studied in this light, because although they are liquid, they hold together in a way that is reminiscent of a solid.

The term coacervate was coined in 1929 by two chemists, Hendrik Bungenberg de Jong and Hugo Kruyt. They were studying the “phenomena of unmixing”, in which two liquids mixed together separate.

An obvious example of this is oil and water, which mix only if you stir them vigorously. Leave them to settle and they will separate – first you will see drops of oil in the water, and eventually all the oil will gather into a distinct layer.

Coacervation is a less absolute form of unmixing. When mixed with water, long-chain molecules like proteins or lipids may choose instead to clump together into droplets. “They’re spherical because they’re still liquid,” says biophysicist Dora Tang at the University of Saarland in Germany. Unlike oil droplets in water, which are 100 per cent oil, “there’s still quite a lot of water inside.”

The crucial thing about coacervates is that they are structurally very simple. Living cells are surrounded by a membrane made of two layers of lipids, arranged in a rather precise way. In contrast, coacervates have no outer membrane, because the molecules aren’t neatly ordered. Tang compares the arrangement of molecules in a coacervate to overcooked spaghetti, the strands of which can get stuck together. There is an “interface” where the coacervate’s outer molecules nudge against the surrounding water, but “no distinct membrane”, she says.

One hundred years ago, scientist Alexander Oparin proposed that these peculiar little droplets were the key to the origins of life. Along with biologist J. B. S. Haldane, he was the founding father of scientific research into the origins of life, working for many decades in what was then the USSR.

Oparin had been writing about how the first life might have formed since the 1920s, and in 1936 he published his magnum opus, The Origin of Life on the Earth. He imagined what Earth might have been like when it was new. In the first oceans – which arose within the first half-billion years after the planet formed – a vast array of chemicals were dissolved or mixed, from fragments of rocks and minerals to simple carbon-based chemicals, all of which started reacting with each other.

The planet became a gigantic chemical factory – one in total chaos. As more complex chemicals formed, the waters became a soup-like mixture, which would eventually be dubbed “primordial soup”. Some of the molecules were, in molecular terms, large, such as proteins and nucleic acids.

These chemicals wouldn’t all stay mixed into the water. Some would separate out, forming coacervates. Oparin was already able to point to experimental evidence that proteins readily did this. These coacervates, Oparin suggested, were precursors to cells. They were nowhere near as complex, and probably not as stable, but they were a crude first attempt at living organisms.

“Oparin did a lot of key conceptual and experimental work with the coacervates, even up to the 1960s,” says Tang.

However, that idea fell by the wayside. There was no evidence of coacervates still playing a role in living cells today. “If we don’t see them in everyday biology, why would they be relevant at the origin of life?” says Job Boekhoven, a molecular engineer at the Technical University of Munich in Germany.

Meanwhile, the molecular biology revolution that began in the 1950s revealed the central importance of nucleic acids like DNA and RNA, so many origins-of-life researchers became obsessed with the creation of genetic material – most famously in the RNA world hypothesis, which proposes that the first life was made of RNA and nothing else.

Likewise, the discovery of the structure of cell membranes in the 1970s led others to try to create simple membrane-bound “vesicles” that could have acted as the first cells. Another school of research focused instead on creating a primordial metabolism: the chemical reactions that build and maintain cells. Coacervates became, for origins-of-life researchers, a historical curiosity.

This changed only when biologists learned that they play key roles in living cells after all. The game-changing discovery came in 2009, when a team led by Anthony Hyman at the Max Planck Institute for Molecular Cell Biology and Genetics in Dresden, Germany, studied so-called P granules, found in cells involved in sexual reproduction. Despite the name, these  “granules” turned out to behave like liquids. A study by another group in 2012 found that many proteins could form such liquid droplets.

We now know that they are ubiquitous in cells – and crucial. For example, in the nuclei of our cells, there is a special region called the nucleolus. It manufactures ribosomes, which are the molecular machines that build all the proteins in our cells. The nucleolus is a cluster of coacervates.

What’s more, it turns out that malfunctioning coacervates are a major health issue. For one thing, they are a key factor in the rapid deterioration of hearts donated for organ transplant. When hearts are chilled to preserve them, some molecules form coacervates. In this state, they activate stress receptors, leading to inflammation and cell death. Last year, researchers used a drug to block the formation of these coacervates and found that the hearts treated this way worked better.

Driving cancer and Alzheimer’s

We now also know that these blobs play a key role in cancer – by helping to activate tumour-promoting genes, for instance, and influencing the response to anti-cancer drugs.

There is growing evidence that coacervates may be involved in the onset of Alzheimer’s disease, too, the most common form of dementia. Proteins called amyloid and tau form plaques in the brain, which are somehow linked to the neurodegenerative symptoms. A string of studies have shown that amyloid proteins can form coacervates and that these in turn seem to cause the proteins to clump.

This explosion of evidence about the importance of coacervates in modern cells, and in human bodies, led some origins-of-life researchers to wonder whether Oparin might have been right after all. The questions were basic: do coacervates really form spontaneously? And how many different biological molecules can self-assemble in this way?

“We found, first of all, [it’s] very easy to form coacervates,” says prebiotic chemist Claudia Bonfio at the University of Cambridge. A lot of previous origins-of-life research has focused on vesicles, which have an outer membrane surrounding a watery centre. These are fiddly to make, she says, whereas coacervates are trivial.

Still, there was an immediate problem. There were plenty of examples of long-chain molecules forming coacervates, says Spruijt. But in the earliest primordial soup, such long molecules would have been rare or non-existent. Most of the carbon-based molecules would have been small.

In 2021, Spruijt and his team showed that this wasn’t a problem. They developed a miniature protein, just four amino acids long, that could assemble itself into coacervates. “This opened a lot of opportunities,” he says. There are plausible scenarios where such relatively simply molecules could form spontaneously and promptly self-assemble into coacervates, he says. The following year, Spruijt’s team showed that another simple peptide called oligoarginine was even more capable, forming coacervates with any of a host of small molecules.

It is difficult to imagine that this didn’t happen if you picture the messy prebiotic soup that could have contained all these building blocks of life, says Bonfio. Coacervates, she argues, probably started forming extremely early, as soon as carbon-based molecules started linking up into even relatively short chains.

Still, so far, these coacervates are little more than empty shells. What could they actually have done?

A core idea, going right back to Oparin, is that coacervates are a way of creating an internal environment. Inside, chemicals could become highly concentrated – unlike the dilute prebiotic soup outside. Neighbouring coacervates can also have different make-ups. “Then you can start driving reactions, and then you can make more molecules, and then you get more diversity [of] molecules in the prebiotic soup,” says Tang. In this way, coacervates can drive greater chemical complexity – pushing the non-living prebiotic soup towards life.

One key process for generating life is stringing together amino acids into chains, known as proteins. Spruijt’s team showed in 2023 that coacervates made of ferricyanide – a simple iron-based compound that was probably in the prebiotic soup – could drive this reaction. There are many other examples. For instance, in 2025, Tang and her colleagues showed that a crucial metabolic reaction – the transformation of the molecule NAD⁺ into NADH – worked up to three times faster in the presence of coacervates, compared with a fully mixed solution. Such findings mean, according to an analysis published in March this year, that coacervates aren’t merely “passive compartments” but instead “active participants” in generating and sustaining life.

There is still a lot to learn about coacervates as reaction centres. While it is clear that molecules can become concentrated within them, we don’t know the systematic rules, says Tang. “Which molecules will go in more than others? We have a rough idea, but we can’t say systematically.”

Likewise, coacervates don’t always speed up chemical reactions. “They can accelerate chemical reactions, they can decelerate chemical reactions,” says Boekhoven. Things can grind to a halt because the interiors of coacervates are very viscous, so molecules move more slowly than they would in water. “It’s all over the place,” he says. “What are the actual rules?”

Kickstarting life

A major challenge is to get some of the key systems found in living cells, like genetics or metabolism, up and running inside coacervates. Tang has shown that some RNAs will accumulate within protein-based coacervates, creating the foundation of a genetic system. She has also built coacervates out of a mix of RNA and proteins, and found that enzymes made of RNA can still work within them.

Bonfio is a fan of this approach and has tried it herself. She is aiming to use the same molecule to both create a compartment and act as a store of genetic information.

But there is a long way to go before coacervates are actually doing something with the genetic material, as opposed to just storing it. “Are there coacervates that actually help RNA replication?” wonders Boekhoven.

Instead, some of the most dramatic advances have come from attempts to make the coacervates themselves behave in a more lifelike way. All living cells can grow by taking in nutrients, and they can reproduce by dividing. Could coacervates do this? A group of theoreticians, including Hyman, suggested as much in a 2016 study. Their simulations indicated that coacervates might grow and divide if they had an external supply of energy.

“I like to build things,” says Boekhoven. So, when he saw this theoretical description of coacervates growing and dividing like cells, his first response was: “Can we build that?”

It took four years, but he and his team managed it. They built coacervates from RNA and proteins, and powered them using a chemical fuel called EDC. When they removed the fuel, the droplets disappeared, but they re-emerged when given more EDC. Furthermore, when the droplets were on the way to decaying, they would shatter into multiple “daughter” droplets.

In line with this, Spruijt’s team has shown that coacervates can grow when given a simple carbon-based fuel. Tang has also found that coacervates can divide if they are housed inside heated rock pores, where bubbles of gas jostle them; such pores have been suggested as cradles for the first life.

There also seem to be ways that coacervates can become more internally complex, until they start to look more like cells. A single droplet can have two or more “compartments” within it, each with subtly different properties that attract different chemicals. Likewise, coacervates can be prodded into forming true membranes, creating a harder barrier against the outside world. Tang’s first study of coacervates, published in 2014, showed that lipids inside a coacervate could self-assemble into a membrane, which would then push its way to the outside and form a covering. A dramatic 2024 study found that simply sprinkling water onto coacervates caused them to form strong chemical bonds along their outer surfaces.

There is a long way to go, because the coacervates in these labs are far simpler than actual living cells and aren’t self-sustaining. But for the researchers who study them, their advantage is that they simplify the problem of the origins of life enormously.

The field has always been riven by chicken-or-egg arguments about which part of life emerged first: metabolism or compartments or replicating nucleic acids or proteins? “I’ve never been a big fan of ‘this was first’ or ‘that was first’,” says Spruijt. “I think the beauty of coacervates is that they can integrate all these different worlds or scenarios together.”

It’s a curious full-circle moment for origins-of-life researchers. One hundred years after Oparin suggested that coacervates might be the key, evidence is accumulating that he had a point. What’s more, the crucial clue wasn’t found in weird microbes in the deep sea, but in our own cells. The keys to the origin of life may have been inside our bodies all along.