I have been interested in abiogenesis for most of my adult life. I was writing about the chemistry of the first cells on this blog twenty years ago, and at the University of Florida I approached a neighboring question from another direction.
I loved an inorganic biochemistry course with Prof. George Christou, and my graduate work in the Richardson group involved transition-metal chemistry and DNA. None of that gives me special authority on the origin of life. It does explain why one feature of a recent Scientific American article about a new origin-of-life study immediately caught my attention: the metals.
The headline says life on Earth may have “emerged twice.” The underlying idea is more interesting. The claim is not that abiogenesis happened twice. It is that the metabolic system inherited from LUCA—the last universal common ancestor—may still have been incomplete.
Some reactions essential to both bacteria and archaea are performed today by unrelated enzymes. That raises an obvious question: if their common ancestor had neither enzyme, what did the chemistry before them?
One answer is geology.
The new work argues that transition metals such as iron, cobalt, nickel, and their alloys could have catalyzed many reactions now handled by enzymes and cofactors, especially in serpentinizing hydrothermal environments. In that model, early metabolism was not entirely inside the organism. Part of it belonged to the surroundings.
That changes the usual picture of the first cell. We tend to imagine a primitive bag of molecules gradually acquiring machinery. But perhaps the earliest evolving systems were chemically incomplete in a more literal sense: they depended on gradients, minerals, surfaces, and metal catalysts supplied by a particular place.
The cell came later.
This makes metals especially interesting. Carbon is extraordinarily good at building structure. Transition metals are extraordinarily good at transferring electrons, changing oxidation states, activating small molecules, stabilizing intermediates, and arranging substrates in useful geometries.
Modern biology still depends on them everywhere. Iron-sulfur clusters move electrons. Nickel appears in hydrogenases. Cobalt sits at the center of vitamin B12. Manganese helps split water in photosynthesis. Zinc and magnesium support enormous amounts of enzymatic and nucleic-acid chemistry.
Life is carbon-based. Its chemistry has never been carbon-only.
The evolutionary transition I find most interesting is what we might call catalytic capture. Imagine useful chemistry occurring on a metal-bearing mineral surface. Organic molecules eventually begin binding those metals. Most interactions would be useless. But some ligands could preserve catalytic activity while changing its solubility, geometry, selectivity, or location.
Now evolution has something it can tune.
The metal remains chemically powerful, but the organic environment increasingly controls how that power is used. A mineral catalyst becomes a metal-organic complex; a metal-organic complex becomes a cofactor; a cofactor becomes embedded in a protein.
The new study does not prove that entire sequence. But it makes the starting point much more plausible: some catalytic work now performed inside cells may once have been performed by the environment.
Seen this way, one major transition in the origin of life was not simply the invention of metabolism. It was the movement of metabolism across a boundary.
A membrane internalizes concentration differences. Enzymes internalize catalysis. Cofactors internalize metal chemistry. Ion pumps recreate gradients that once came free from geology.
What had been supplied by place became portable.
Perhaps that is what cellular autonomy means. Life did not suddenly separate itself from geochemistry. It gradually captured pieces of the environment, enclosed them, regulated them, and eventually encoded the machinery needed to reproduce them.
The first cells may not have invented metabolism.
They may have learned how to carry its catalysts with them.
Related Posts from the Archive
Summary of abiogenesis breakthroughs
June 15, 2009
“Following up on an item from last month, the NYT has a great summary of the four principal features of new origins-of-life models...” The post also highlights hydrothermal-vent proposals involving “gases and metallic catalysts” and the emergence of the first metabolic processes.
Summary of origin of life research
July 5, 2008
“I like to write things about abiogenesis. Go here for some great peer-reviewed publications on the origin of the genetic code and the evolution of the chemical processes underlying metabolism.”
Abiogenesis in ice
April 22, 2008
“Of all the recent things I've read about abiogenesis, this is one of the better ones from a chemist's perspective.” A look at how temperature, concentration, phase behavior, and eutectic freezing can change the chemistry available to prebiotic systems.
On the Origin of the Genetic Code and Abiogenesis
March 16, 2007
“A new research paper in Science demonstrates yet more evidence for abiogenesis”—followed by a discussion of ribozymes, the RNA world, the origin of the genetic code, and stepwise transitions from nonliving to living chemical systems.
Abby and Her Five Senses
December 13, 2005
“Recently I've had abiogenesis on my mind.” The post turns toward the transition from organic chemistry to extant metabolism and an early interest in stepwise models connecting nonliving chemistry to life.
