20.5: Organic Molecular Origins of Life Closer to Home

Deep in the oceans, far from the meteoric bombardments and the rampant free energy of an oxygen-free and ozone-less sky, deep-sea hydrothermal vents would have been spewing reducing molecules (e.g., \(\rm H_2S, H_2, NH_4, CH_4\)), much as they do today. Some vents are also high in metals such as lead, iron, nickel, zinc, and copper. When combined with their clay or crustal substrata, some of these minerals could have provided catalytic surfaces to enhance organic molecule synthesis. Could such localized conditions have been the focus of prebiotic chemical experimentation leading to the origins of life? Let’s look at two kinds of deep-sea hydrothermal vents: volcanic and alkaline.

20.5.1 Origins in a High-Heat Hydrothermal Vent (Black Smoker)

The free energy available from a volcanic hydrothermal vent would come from the high heat temperatures ranging to \(350^{\circ} \rm C\)) and the minerals and chemicals expelled from the Earth’s mantle. Figure 20.8 (below) shows a volcanic hydrothermal vent.

Figure 20.8: An oceanic volcanic hydrothermal vent, or black smoker

Conditions assumed for prebiotic volcanic hydrothermal vents could have supported the catalytic synthesis of organic molecules from inorganic precursors (see Volcanic Vents and Organic Molecule Formation) or Volcanic Vents & Organic Molecule Formation-full paper). Available catalysts would have been inorganic, for example metals like nickel and iron. Chemical reactions tested include some that are reminiscent of biochemical reactions in chemoautotrophic cells alive today. Günter Wächtershäuser proposed the Iron-Sulfur World Theory of life’s origins in these vents, also called “black smokers”. These vents now spew large amounts of \(\rm CH_4\) and \(\rm NH_4\) and experiments favor the idea that iron-sulfur aggregates in and around black smokers could provide catalytic surfaces for the prebiotic formation of organic molecules like methanol and formic acid from dissolved \(\rm CO_2\) and the \(\rm CH_4\) and \(\rm NH_4\) coming from the vents. A variety of extremophiles (e.g., thermophilic archaea) now living in and around black smokers would seem to be testimony to black smoker origins of life.

Wächtershäuser also realized that prebiotic selection acted not so much on isolated chemical reactions, but on aggregates of metabolic reactions. We might think of such metabolic aggregates as biochemical pathways or multiple integrated biochemical pathways. Wächtershäuser also proposed the selection of cyclic chemical reactions that released free energy that could then be used by other reactions.

Some version of this prebiotic metabolic evolution, or selection of metabolic chemistries, something more than a simpler chemical evolution, would have been essential to the origins of life.

CHALLENGE

Does co-selection of multiple metabolic pathways (i.e., “aggregates of metabolic reactions”) seem feasible? Can you think of a metabolic aggregate that might illustrate co-selection? Explain.

While the idea of selecting metabolic pathways has great merit, there are problems with a life-origins scenario in volcanic hydrothermal vents. For one thing, their very high temperatures would likely have destroyed as many organic molecules as were created. Also, the extremophilic archaea now found around these volcanic vents cannot be the direct descendants of any cells that might have originated there. Woese’s phylogeny clearly shows that archaea share a lineage with eukaryotes (not eubacteria). Thus, extremophilic cellular life originating in the vents must have given rise to a more moderate LUCA first, before then going extinct. Then, extremophiles we find in the black smokers would once again have had to evolve independently to re-colonize the vents! Such twists and turns militate against an extremophiles-first origins scenario. Given these concerns, recent proposals focus on life origins in less extreme alkaline hydrothermal vents.

20.5.2 Origins in an Alkaline Deep-Sea Vent (White Smoker)

Of the several scenarios discussed here, an origin of autotrophic life in alkaline vents is one of the more satisfying alternatives to a soupy origin of heterotrophic cells. For starters, at temperatures closer to \(100^{\circ} \rm C-150^{\circ} \rm C\), alkaline vents (white smokers) are not nearly as hot as are black smokers. An alkaline hydrothermal vent is shown below in Figure 20.9.

Figure 20.9: An alkaline deep-sea hydrothermal ocean vent, or White smoker

Other chemical and physical conditions of alkaline vents are also consistent with an origins-of life scenario dependent on metabolic evolution. For one thing, the interface of alkaline vents with acidic ocean waters has the theoretic potential to generate many different organic molecules [Shock E, Canovas P. (2010) The potential for abiotic organic synthesis and biosynthesis at seafloor hydrothermal systems. Geofluids 10 (1-2):161-92)].

In laboratory simulations of alkaline vent conditions, the presence of dissolved \(\rm CO_2\) favors serpentinization, a reaction of water and heat with serpentinite, an iron-containing mineral found on land and in the oceanic crust. Figure 20.10 is a sample of serpentinite.

Figure 20.10: Serpentinite from Deer Lake in upper Michigan is a mineral also found in the oceanic crust that under conditions found in an alkaline vent, can form methane from \(\rm CO_2\).

Experimental serpentinization produces hydrocarbons, and a warm aqueous oxidation of iron produces \(\rm H_2\) that could account for abundant H2 in today’s white smoker emissions.

Also, during serpentinization, a mineral called olivine [(\(\rm Mg^{+2} , \rm Fe^{+2})_2SiO_4\)] reacts with dissolved \(\rm CO_2\) to form methane (\(\rm CH_4\)). So, the first precondition of life’s earthly origins, the energetically favorable creation of organic molecules, is possible in alkaline vents.

Proponents of cellular origins in a late-Hadean non-reducing ocean also realized that organic molecules formed in an alkaline (or any) vent would disperse and be rapidly neutralized in the wider acidic ocean waters. Somehow, origins on a non-reducing planet had to include a way to contain newly formed organic molecules from the start, and a way to power further biochemical evolution.

What then, were the conditions inside an alkaline vent that could have sequestered organic molecules and led to metabolic evolution and ultimately, life? Let’s consider an intriguing proposal that gets at an answer!

The porous rock structure of today’s alkaline deep-sea vents provides micro-spaces or micro-compartments that might, in a prebiotic past, have captured some of the alkaline liquids they emitted. In fact, conditions in today’s white smokers also support the formation of hydrocarbon biofilms, primitive organic membranes that could have been lined those porous rocky micro-compartments. Alkaline vent emissions would then be trapped within primitive biofilm membrane vesicles enclosed by rocky “cell walls”. An interesting consequence of this scenario would be the existence of a natural proton gradient between the alkaline contents of the micro-compartments and the surrounding acidic ocean waters. Did all this happen?

Perhaps! Without a nutrient-rich environment, heterotrophs-first is not an option. That leaves only the alternate option: an autotrophs-first scenario for the origins of life. Nick Lane and his coworkers proposed that the natural proton gradients suggested above were the selective force behind the evolution of early metabolic chemistries in alkaline vents ( Prebiotic Proton Gradient Fuels Origin of Life). Organized around biofilm compartments, prebiotic structures and chemistries would have harnessed the free energy of these proton gradients. In Lane’s view, the first protocells, and therefore the first cells, may have been chemoautotrophs. If so, ask yourself if early autotrophy could have evolved without an electron transport system!

Last but not least, how might chemoautotrophic chemistries on a non-reducing planet have supported polymer formation, as well as polymer replication? Today we see storage and replication of information in nucleic acids as separate from enzymatic catalysis of biochemical reactions. But are they all that separate? If replication is the faithful reproduction of the information needed by a cell, then enzymatic catalysis ensures the redundant production of all molecules essential to make the cell! Put another way, if catalyzed polymer synthesis is the replication of the workhorse molecules that accomplish cellular tasks, then what we call ‘replication’ is nothing more than the replication of nucleic acid information needed to faithfully reproduce these workhorse molecules.

Was there an early, coordinated, concurrent selection of mechanisms for the catalyzed metabolism as well as catalyzed polymer synthesis and replication? We’ll return to these questions shortly, when we consider the origins of life in an RNA world.

An origin of life scenario in a non-reducing (and oxygen-free) atmosphere raises additional questions. Would proton gradients provide enough free energy to fuel and organize life’s origins? If so, how did cells arising from prebiotic chemiosmotic metabolism harness the energy of a proton gradient? Before life, were protocells already able to transduce gradient free energy into chemical free energy? Was ATP selected to hold chemical free energy from the start? Or was it selected only concurrent with metabolic selection? Alternatively, was the relief of the gradient coupled at first to the synthesis of other high-energy intermediate compounds, with for example, thioester linkages? Later, how did cells formed in alkaline vents escape the vents to colonize the rest of the planet?

However, the energy of a natural proton gradient would be initially captured, the chemoautotrophic LUCA must already have been using membrane-bound proton pumps coupled to electron transport, and a membrane ATP synthase to make a proton gradient and then harness its free energy to make ATP, since all of its descendants do so.

Finally, when did photoautotrophy (specifically oxygenic photoautotrophy) evolve? Was it a late evolutionary event? Is it possible that photosynthetic cells evolved quite early among some of the chemoautotrophic denizens of the white smokers, biding their time before exploding on the scene to create our oxygenic environment?

351 Life Origins in a Thermal Vent