How could life emerge on a previously lifeless planet? Is life’s origin a chance event, or a stepwise process? Is it likely that worlds other than Earth have also come up with life; and if so, which ones?
After many frustrations and false starts, developing lines of research are finally offering answers to these questions.
A recent paper in the scientific journal Astrobiology concludes — subject to further testing — that life could be produced via a step-by-step process in and around hot springs on a volcanic island about four billion years ago.
Like earlier theories of life’s origin, the described process begins with simple chemical building blocks forming in an atmosphere quite different from Earth’s atmosphere today.
A wet-dry cycle — waters evaporating and returning — is an important part of the process.
Evaporation causes the chemical building blocks to clump together into cell-like structures with oily membranes (consisting of lipids) around a moist interior (consisting of water and various carbon compounds). Each is like a tiny test-tube, doing its own experiment.
They are vastly simpler than living cells as we know them. Nonetheless they go through a form of natural selection, when returning waters cause comparatively ramshackle structures to break up while sturdier ones survive. In the driest part of the cycle, the cell-like structures are partially merged, allowing the results of successful experiments to be shared.
This is the crude beginning of what eventually becomes Darwinian evolution.
The paper is by Bruce Damer, a Canadian-American systems theorist, and David Deamer, an American biologist. They draw from the work of various other researchers, including
- Australian astrobiologists Tara Djokic and Martin van Kranendonk, who investigated ancient mineral formations (stromatolites) in what is now the Pilbara region of Western Australia. These formations are consistent with early microbial life in a region of land hot springs.
- British biophysical chemist Marc Rodriguez-Garcia, who investigated the ability of evaporating water to link together animo acids into peptide chains.
- Israeli geneticist Doron Lancet, who proposed the concept of GARD (graded autocatalysis replication domain), a form of rough-and-ready evolution that does not require a genetic code.
- British organic chemists Matthew Powner and John Sutherland, who developed the idea of “Goldilocks chemistry” — a family of reactions which can generate just the right molecular building blocks to form life as we know it.
Out of various environments available on early Earth (ocean vents, tidal pools, land hot springs), Damer and Deamer argue that land hot springs are the sites where Goldilocks chemistry could occur most readily, as they would have had comparatively low concentrations of mineral salts, and comparatively high concentrations of carbon compounds from the atmosphere. This is because their waters come from rain that soaks into the ground, meets hot rocks, then flows or geysers above the surface.
The authors of the paper don’t conclude that Earth must have given birth to life in exactly this way they describe. Their conclusion, which is subject to further testing, is that life could have arisen on Earth via this path, and could also arise on other worlds in a similar way, as long as the start conditions were there.
The new thinking is revolutionary in the sense that it challenges two widely accepted ideas:
- the idea that life began in the sea
- the idea that self-replicating molecules appeared before cells
Neither of these ideas has been proven; but to eminent scientists working on this topic, both have seemed like natural assumptions to work with.
Both assumptions are found in the work of Britain’s J.B.S.Haldane (1892 to 1964). The American biochemist Norman Horowitz (1915 to 2005) argued for life arising “as individual molecules’’ at the First International Symposium of The Origin of Life on the Earth, held in Moscow in 1957.
These two working assumptions have been like the obvious suspects in a murder mystery, distracting attention from other possibilities.
On the other hand, the new thinking confirms some long-standing suspicions.
For instance, the evaporation stage of the wet-dry cycle described by Damer and Deamer resembles crystallisation of salt. Which calls to mind an intuition that goes back over 90 years: the idea that growth of living matter is in some ways comparable to growth of crystals.
This idea of life as crystal was developed in the first half of the twentieth century by two notable theorists: by the Austrian-Irish physicist Erwin Schrödinger (1887 to 1961), in his book What is Life , published in 1944; and before that by the Soviet biochemist Alexander Oparin (1894 to 1980), in his Origin of Life, published in 1924.
To work out how much life there is in the universe, we need to know two things.
First, how many worlds could support life? Second, out of the worlds that could support life, what proportion actually get it?
There are theories about the origin of life which involve an event of extremely low probability. French biochemist Jacques Monod wrote fifty years ago:
The universe was not pregnant with life… Our number came up in the Monte Carlo game. (Chance and Necessity, p 145)
It has been seriously suggested that the probability of life’s number coming up may be so low that it’s unlikely to have occurred anywhere else in the observable universe, of hundreds of billions of galaxies.
This is the working assumption of Japanese astronomer Tomonori Totani. In a paper published February this year, he argues that there is a significant probability of life emerging by chance, on very rare planets, only if the observable universe is a small part of a even larger universe: a vastly larger one. According to Totani, the existence of this super-universe is consistent with the theory of cosmic inflation.
Notions like this derive from the idea that the intricate genetic machinery of living cells today is a necessary precondition for evolution in any form, rather than an outcome of earlier forms of evolution.
Although Jacques Monod was a great biochemist and Tomonori Totani knows a lot about cosmology, the emerging picture of life’s origin seems to show that the assumptions they’ve worked with were wrong. A low-probability event is not the most probable explanation for the fact that Earth has life.
Any planet with start conditions like Earth’s, including similar chemistry, volcanic islands and hot springs, is likely to get a similar result — a population of microscopic living things. Some of these may evolve into bigger living things later on, though what proportion will do so is another question.
However, the new model does not mean that life will spring up everywhere there is liquid water.
As Damer and Deamer point out, an evaporation-cycle path to living cells would not be available on a world such as Europa, whose atmosphere is too thin for water in liquid form on its surface, no matter how much liquid water it may have underneath its surface ice.
Life could feasibly to have emerged from evaporation cycles on Mars four billion years ago, when that planet had a thicker atmosphere and surface water.
Damer and Deamer don’t discuss Titan in their recent paper. However, the Cassini mission established that Saturn’s largest moon has a thick atmosphere rich in carbon compounds, plus bodies of surface liquid which go through cycles of evaporation and replenishment; although the liquid consists of methane and ethane rather than water and is by Earth standards extremely cold.
The results of wet-dry cycles in Titan conditions may be an interesting topic for scientific modelling, and eventual direct study by space probes.
Looking beyond this solar system, and considering what we now know about the proportion of stars that have planets, the galaxy’s overall population of living things, as implied by the new model, must be literally astronomical.
In short, it looks like the universe is fertile with life.