On early earth, a little heat could have led to more complex life

Before true cells existed on the earth, organic molecules floated freely in water. The first cell membrane created a compartment so that useful molecules like RNA and proteins could stay close enough to interact with each other, leading to more complex biological functions.

However, an important question remains: how did the first protocells on early earth bring together all the molecules they needed and set life-like chemistry in motion? According to a new study, simple heat, like the warmth of volcanic rocks, could have done the trick.

Putting ingredients together

When one side of a small water-filled crack is warmer than the other, two things happen. Warm liquid rises and cooler liquid sinks, creating a gentle loop; second, many molecules drift from the hotter side towards the cooler side. Together, these flows can sweep dissolved molecules downwards and hold them there.

The study, authored by scientists from Canada, Finland, Germany, and Italy, was published in Nature Physics.

The scientists built small 170-micrometre-thick chambers sandwiched between sapphire plates. The top plate was maintained at 40º C and the bottom plate at 27º C.

Then they turned to PURExpress, a cell-free protein synthesis kit made from Escherichia coli bacteria. The kit contains every major part of E. coli’s protein-making machinery — DNA, RNA polymerase, amino acids, etc. — in purified form.

Before the experiment, the team diluted these contents threefold in order to keep the kit from being able to make proteins.

Next, they added a short piece of DNA that coded for a protein called green fluorescent protein (GFP) to each PURExpress mix. GFP fluoresces with a bright green light that can be seen under a microscope. As a result, the mix had a built-in light bulb that reported when and where protein synthesis happened.

The authors let the PURExpress mix ‘express’ itself for around 16 hours both with and without the temperature gradient between the sapphire plates.

Then they opened a narrow channel at the top and pumped pure water for up to nine hours or a nutrient feed for up to 22 hours while maintaining the gradient.

Right after, the team froze the chambers to preserve their concentration profiles for later study. Then they split the frozen sample into three layers from top to bottom and analysed each slice.

Membrane-like sans a membrane

They found that there was 25-times more GFP in the bottom layer than in the top. Similarly, key ions including those of magnesium (30x) and potassium (7x) and phosphate ions (70x) had accumulated more at the bottom than at the top. They team also found that DNA, RNA building blocks, and amino acids had become concentrated at the bottom.

Once these molecules were crowded together, the previously inactive PURExpress mix had switched on gene expression. The team found that the mix was manufacturing GFP only in the chamber with the temperature gradient, not in the chamber without. In fact, even when water flowed overhead for nine hours, more than 95% of the GFP was trapped while small amounts of phosphate waste diffused out, displaying membrane-like selectivity without an actual membrane.

For added measure, the team also modelled the heat, flow, and diffusion and found that they reproduced the 3D concentration profiles of various molecules.

Thus, according to the study,just a rock crack exuding heat could have gathered different types of biomolecules together and kickstarted protein synthesis.

Over time, cell membranes let early cells set up ion gradients, i.e. different ion concentrations inside versus outside. When ions flowed back through primitive channels, the flux could power the first molecular machines.

Keep it simple

The researchers wrote that the phenomena they’ve proposed could be playing out around hydrothermal vents. This will need to be checked.

National Centre for Biological Sciences professor Shashi Thutupalli also said the phenomena described in the study “would rely on some steady gradient. Whether the timescale of the temperature gradients in nature are similar to those in the study needs to be checked.”

He also said he was curious whether all kinds of molecules would move in response to the temperature gradient.

“In my opinion, I don’t think we’ll ever exactly figure out what exactly happened on early earth. But one takeaway is that maybe the start of life needn’t have been very complicated or specialised,” Dr. Thutupalli said.

For example, a March 2025 study in Science found that when neutral water is sprayed, it creates oppositely charged microdroplets that cause an electrical discharge, instigating chemical reactions around them.