Have you ever wondered if you could kick-start life on another planet? In the Origins of Life laboratory at McMaster University in Canada, there is a machine that allows you to try this very task.
Exactly how life began on the Earth remains heavily debated, but one of the most famous ideas was proposed by Charles Darwin in a letter to a friend in 1871:
“But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts…” Darwin began.
In contrast to the vast ocean, a pond would allow simple organic molecules to be concentrated and increase the probability of reactions that would form chains of longer molecules such as RNA; a single-stranded version of DNA that is thought to have been used for genetic information by the earliest forms of life.
It is highly likely that such warm little ponds would have the necessary ingredients to build such complex molecules. Experiments performed by Stanley Miller and Harold Urey in the 1950s demonstrated that water containing just the basic molecules of methane, ammonia and hydrogen would react to form a wide range of simple organics. Meteorites have also been found to contain similar molecules, proposing an alternative way of populating pools of water on the early Earth.
These ponds should therefore contain plenty of simple organics such as nucleotides, which stack together to form RNA. However, this stacking step turns out to be tricky.
“Anywhere where you have stagnant water and take sample, you will find organic molecules,” explains Maikel Rheinstädter, associate director of McMaster’s Origins Institute. “But you only find the building blocks, not the longer chains. Obviously, something is missing.”
In pond water, molecules are free to move around and potentially meet to initiate a reaction. The problem is that nucleotides carry a negative charge which repels the molecules from one another. While their motion is unconstrained, the nucleotides will therefore not approach close enough to react and form a longer molecule.
The solution is to dry out the pond.
As winter turned to summer on our young planet, shallow pools would have evaporated to leave the molecules suspended in the water lying on the muddy clay bottom. Clay also carries a slight electric charge, cancelling out the negative repulsion between nucleotides. Should the two nucleotide molecules land close to one another, they would be in the perfect position to react and form a strand of RNA.
Molecules that did not fall to the pond floor in the right orientation for reacting would remain as simple nucleotides through the dry months. However, as winter comes once more and the pond refills, the molecules are once again free to move and land in different positions when the water next dries out. Through repeated wet and dry cycles, Rheinstädter explains, the quantity of RNA could gradually increase.
It is this process of wet and dry cycles in warm little ponds that is mimicked by the Origins Institute’s “Planet Simulator”. The semi-circular chamber can reproduce conditions on the early Earth or under the starlight on a more alien terrain.
“If anyone has an idea they want to test, they can come in here and run the simulation,” enthuses Rheinstädter.
The Origins Institute opened in July 2004 with the goal to address how stars, planets and life began in the Universe. Graduate students come from backgrounds that span the natural sciences, bringing expertise from biology through to chemistry, geology and physics. Such interdisciplinary research is a speciality of McMaster University, which is also home to the internationally award-winning Integrated Science Program (iSci) for undergraduates who wish to integrate concepts and methods across different disciplines.
To kick-start your own life on a new world, a smudge of nucleotides and clay is placed on a silicon chip. Silicon is an unreactive substance that prevents the chip interfering with the experiment. Once placed inside the machine, you can vary the temperature, composition of the atmosphere, pressure, humidity and wavelengths of light that will bathe your chip-sized warm pond starter kit.
“We’re focussed on exploring water-based life,” Rheinstädter explains. “So we can operate between temperatures just below freezing at about -20°C [-4°F] to just past boiling.”
To produce rain, the humidity within the chamber is adjusted so that water vapour in the air condenses into a liquid or snowflakes. This avoids the need to open the chamber during the experiment.
Rather than wait years for repeated changes of seasons on Earth, the Planet Simulator cycles rapidly through wet and dry periods. A year on a planet surface can pass in about a week within the chamber. As reactions between molecules occur almost instantaneously, this accelerated rate can accurately reproduce the outcome in a pond experiencing longer seasons.
The chamber is illuminated by LED lights that can vary the wavelength of radiation shining on the chip of pond material. The pond can therefore experience sitting under the dim red glow from a star such as TRAPPIST-1 or Proxima Centauri or try to build complex molecules during a period of the early Earth’s history where the thin atmosphere would have let through more ultraviolet light than today.
The chamber also contains a web camera that live broadcasts simulations on the Planet Simulator’s own YouTube channel.
The Planet Simulator began exploring these new worlds last October and the results have been far from expected.
After 70 days in the Planet Simulator, not only had strands of RNA been created, but other molecules had spontaneously formed distinct regions surrounded by a membrane. These are structures that strongly resemble biological cells. What is more, the RNA had moved to sit within these cells in the same way that our own genetic material is stored in the cells of our body.
“We did not do anything to this,” emphasises Rheinstädter. “It’s just a chip with simple organics and we ran the cycle. Nothing else.”
The appearance of a such a characteristic biological structure looks like the first step in evolution, but Rheinstädter is swift to explain that we are looking at a system that has not yet begun biology.
“This is a system that is determined entirely by physics and chemistry,” he says. “At a later point, there may be a transition into a functioning biological system. But here? This structure is driven only by the properties of these molecules.”
The membrane is formed from molecules that contain both water-loving (hydrophilic) and water-hating (hydrophobic) segments. By forming an enclosed barrier, the water-loving side of these molecules can point outwards to face the wet exterior of the resultant cell, while the water-hating segments are shielded inside. This explains how the properties of these molecules form a cell, but why does the genetic material move inside?
“If you have an answer to that question, you should learn Swedish because they will call you for your Nobel Prize!” Rheinstädter confides. It seems that the Planet Simulator has not yet revealed all its secrets.
So what does this mean for the potential for life elsewhere in the Universe? Rheinstädter notes how rapidly these first non-biological cells can form.
“This process is likely happening millions of times, every second, right now,” he points out. “If you count the number of potentially habitable planets —even if they’re just a small fraction in the planetary systems out there— we’re still talking millions or billions of worlds like Earth. How likely is it that this process is happening right now on those planets? It must, just by the probabilities. There must be planets out there where this is happening.”
But if this process of early cell formation is so easy, the question is why the Earth did not begin to form life until four billion years ago? The answer must be that not all of these cells are able to later become biologically functioning cells.
“It’s like text you are trying to read at a distance,” Rheinstädter suggests. “It looks like text, but it could be just a random collection of letters. Some of the molecules we’re making in this process may have no biological function.”
The exact types of molecules produced in the Planet Simulator also depends strongly on the conditions selected within the chamber. As nucleotides stack to produce strands of RNA, different ponds will produce different sequences. Some of these may be suitable for life, but they will not necessarily be life as we know it here on Earth.
The warm little ponds are clearly very good at making complex molecules, but the question remains about what is needed for biological life.
“The chemistry and the physics are easy. But the biology is a different thing,” concludes Rheinstädter.
