Origin of life: will we ever know how it really began?

The origin of life can increasingly be investigated in the laboratory. Researchers can reproduce aspects of early Earth and watch simple chemistry generate increasingly complex molecules. Yet this creates an unusual scientific problem: even if a laboratory eventually produces something we would confidently call alive, that would not prove that nature followed the same route nearly four billion years ago.

Scientists are trying to reconstruct an event that left no complete record. Earth's surface has been repeatedly transformed since then, erasing much of the physical evidence.

Some broad pieces of the story are nevertheless becoming clearer. Widely accepted fossils show that life existed at least 3.4 billion years ago, and its beginnings may reach considerably farther back. Most origin-of-life researchers also do not envision a single instant in which nonlife suddenly became life. There was probably a long sequence of intermediate chemical systems.

Reconstructing that sequence is the central challenge.

What had to come first?

A modern cell depends on several capabilities at once. It needs to acquire and use energy, store and copy information, and maintain a boundary separating itself from its surroundings.

Which of these appeared first?

One influential possibility is the RNA world. RNA is used by modern cells in genetic information processing, but some RNA molecules can also catalyze chemical reactions. That makes RNA an attractive candidate for an early molecule that could have combined information storage with chemistry needed for replication.

There is a major difficulty, however. Producing modern functional RNA through plausible prebiotic chemistry is not straightforward, and RNA as we know it is relatively fragile.

That raises another possibility: the earliest hereditary system may have been much messier than its modern descendant.

At McMaster University in Canada, researchers use a device called the Planet Simulator. Its chamber can reproduce different temperatures, atmospheric compositions, wet-dry cycles and radiation environments intended to approximate conditions on early Earth or other worlds.

In simulations of ancient Earth, physicist Maikel Rheinstädter and colleagues have observed molecules joining into RNA strands roughly 100 bases long. The strands contain structural defects and do not resemble pristine laboratory-made RNA, but they are remarkably robust.

This is not evidence that such molecules actually served as the genetic material of early life. The researchers are still investigating whether the imperfect strands can perform crucial functions such as catalysis and information transmission.

But the result suggests a different way to frame the problem. Perhaps researchers should not expect modern biological molecules to have appeared fully formed. Early chemical systems could have been rough precursors that evolution subsequently refined.

A different family of ideas puts metabolism first. In these scenarios, networks of chemical reactions capable of sustaining and expanding themselves preceded sophisticated hereditary molecules. Information systems would have emerged later.

The actual history might also have combined elements of both. “RNA first” and “metabolism first” need not represent two mutually exclusive histories.

Where could chemistry have become biology?

The location of life's origin is just as contentious.

Having the right ingredients is not enough. Molecules often need to become concentrated and experience particular physical conditions before useful reactions can proceed. A vast ocean creates an obvious difficulty because potentially important ingredients can simply become diluted.

This is why researchers are interested in environments that repeatedly concentrate chemicals, including shallow bodies of water undergoing cycles of evaporation and refilling.

Soda lakes are particularly intriguing. These highly alkaline, carbonate-rich waters can accumulate phosphate, an element central to nucleic acids, energy metabolism and many other processes in life today.

One striking example is Last Chance Lake in Canada. Water evaporates during the summer, concentrating dissolved substances, and later returns. Such wet-dry cycles could create chemical opportunities that are difficult to reproduce in the enormous volume of an ocean.

The origin-of-life question therefore involves more than asking which molecule appeared first. It also requires explaining where the necessary ingredients could have encountered one another at useful concentrations.

Making life would not automatically reveal history

This leads to the deepest problem.

Suppose scientists eventually reproduce plausible early-Earth conditions. Simple compounds begin generating more complex molecules. Hereditary chemistry, energy-processing reactions and compartments emerge. Eventually the system can reproduce and evolve.

That would be an extraordinary result. It would demonstrate an experimentally viable transition from nonliving chemistry to biology without requiring preexisting life.

But one historical question would remain: did Earth actually follow that route, or would the experiment have revealed only one of several possible routes?

Researchers disagree.

One view is that physics and chemistry constrain the possibilities so strongly that similar environments repeatedly drive matter toward similar solutions. If that is true, reconstructing the transition in the laboratory could bring us close to the actual mechanism by which life arose.

The more cautious view is that multiple chemically plausible pathways may exist. We might then learn in considerable detail how life can begin without ever knowing exactly which sequence occurred on Earth.

Life elsewhere could provide an unexpected test. If independently originated organisms on worlds with similar histories use comparable biochemical principles, that would suggest that chemistry has preferred routes toward biology. Radically different biochemistry would instead point toward a much larger space of possible solutions.

The origin of life is therefore becoming increasingly experimental. Scientists can produce many biological building blocks, study self-assembling membranes, simulate ancient environments and investigate possible precursors to hereditary systems.

The harder boundary lies beyond the individual pieces: how a collection of molecules became a system capable of maintaining information, exploiting energy, reproducing and entering Darwinian evolution.

Chemistry crossed that boundary at least once on Earth. The unresolved question is whether nature had one route across it—or many.