A lot of research has gone into understanding the beginnings of life on Earth 3.8 billion years ago,
and much is still shrouded in mystery.
We know, based on the geological record, that life began as simple single-celled organisms.
Scientists have created the molecules of life in the lab by blasting simple precursors with
radiation or simulated lightning. Amino acids, sugars, nucleotides have all been created
under artificial conditions attempting to mimic the early Earth environment.
But wait a minute. What’s the point in creating these exotic “molecules of life�
If they are not interacting with each other in useful ways?
They could all be swept away by the next wave or wind gust.
The molecules of life must be kept together in a confined space to do the things that living
organisms do--create proteins, undergo metabolism, reproduce! ALL life forms have a barrier—a cell
membrane. It is the most fundamental component of living systems. After 2 billion years, gargantuan
cells, the eukaryotes, evolved which gave membranes within membranes—compartmentalization!
Making many cellular processes more efficient. Another billion years and multi-cellular
systems emerged, but the cell remained the single irreducible entity of every living
thing. How did the cell structure originate? First, let’s understand what a cell membrane is.
A cell membrane is composed of a lipid bilayer with water on the inside and outside.
It is thought that the earliest life forms made membranes from single chain amphiphiles.
An amphiphile is hydrophilic at one end and is most stable interacting with water, at the other
end is a hydrocarbon chain which is lipophilic (or oil-loving). The amphiphiles spontaneously line up
so that the lipid ends are away from the water and the hydrophilic ends are exposed to the
water. Under the right conditions, these will form protocells. It has been a puzzle how amphiphiles
formed in the early earth and made protocells, but new research has shed light on the answer.
To understand, we have to plunge into the ocean depths of the early Earth.
Deep, deep underwater. At thousands of meters underwater,
alkaline hydrothermal vents spring up at the rifts dividing tectonic plates where ocean water seeps
into the earth’s mantle and then is released from vents at high temperature and pressure.
The expelled alkaline water deposits mineral-rich chimneys up to 60 meters
tall. Where the vent water, the surrounding sea water and the mineral deposits come together
is where the chemistry takes place that may hold the answer to the origins of life.
The fluid flowing out of the vent is divided by the surrounding sea water by a thin layer
of iron nickel sulfide rich minerals which have crystallized, forming the vent chimney.
The vent fluid is hot, it is alkaline, and it has high levels of dissolved molecular
hydrogen. The surrounding sea water, on the other hand, is cold, acidic and has
high levels of dissolved carbon dioxide. This arrangement creates an electrochemical reactor
in which hydrogen and CO2 react. The hydrogen serves as a reductant, donating electrons to
carbon dioxide. The iron mineral layer acts as a semiconductor, shuttling electrons across the
interface. The reduction of carbon dioxide forms formic acid which is the starting point for the
creation of lipid bilayers. This process has been duplicated under laboratory conditions.
Under the hot, hydrothermal vent conditions the formic acid decomposes to give carbon monoxide.
The components will react, catalyzed by iron, to the single chain amphiphiles, giving both
fatty acids and long-chain alcohols, in various chain lengths. This process has been demonstrated
in the lab which mimics the conditions at hydrothermal vents. The amphiphiles will line up
to give their most stable configuration and form lipid bilayer structures. It has also been shown
that the conditions at the hydrothermal vents (namely, high pH and high salt concentration)
favor the formation of spherical protocells. These can encapsulate and retain a dye for over 24 h,
establishing that the membrane creates a stable barrier from the outside environment.
If the protocell is large enough, it will pinch off or “bud” forming a daughter cell. This is not a
living system, but it does represent an essential component of all life as we know it.
Hydrothermal vents have generated a lot of excitement in recent years as a starting
place for life on Earth. It is a dynamic system with continuous flow of chemical precursors, plus
energy, plus mineral catalysts to trigger chemical reactions. Hydrothermal vents are still found on
the ocean floor today, and they are teeming with life. Hydrothermal vents also pose the intriguing
question of whether life could have started at other locations in our solar system.
Europa, one of Jupiter’s moons, and Saturn’s moon, Enceladus, are home to vast under-ice
oceans that may contain hydrothermal vents similar to those found on Earth.
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