Geology Reference
In-Depth Information
alcomplexityofmineral-richseawater,ortheprotectivesurfacesofrocksonwhichbiomo-
lecules are now known to bind. Nonetheless, the origins field has now moved beyond the
Miller-Urey scenario, and for many experts, Earth's deep dark zones are today the primary
focus in the biogenesis game.
As I mentioned earlier,
every
ancient environment with sources of energy and small
carbon-bearing molecules probably produced its share of amino acids, sugars, lipids, and
other molecular building blocks of life. An atmosphere laced with lightning or exposed to
harsh radiation remains in the running as a theory of biogenesis; so do black smokers and
otherdeep,hotenvironments.Biomoleculesformduringasteroidimpacts,onsun-drenched
dust particles high in the atmosphere, and in deep-space molecular clouds exposed to cos-
mic rays. Every year tons of organic-rich dust rain down on Earth's surface from outer
space, as it has for more than 4.5 billion years. We now know that life's building blocks
litter the cosmos.
Step 2: Selection
Ahalf-centuryago,thegreatest challenge inoriginsresearchwassynthesizing therawma-
terials:themolecularbricksandmortaroflife.Bythedawnofthetwenty-firstcentury,that
problem had largely been solved; scientists realized that Earth must have been girdled by a
dilute consommé of life's vital ingredients. Much of the focus has now shifted to the selec-
tion, concentration, and assembly of biobits into macromolecules—to the membranes that
enclose thecell, theenzymes that promote itschemical reactions, andthegenetic polymers
that pass information from one generation to the next.
Two complementary processes likely played a role. One is self-assembly, in which a
group of elongated molecules—lipids—clumped together spontaneously to form the mem-
branesthatencapsulated thefirstcells.Lipidsfeatureskinnybackbonesofadozenormore
carbonatoms.Undercertainconditions,theytendtoself-assembleintomicroscopichollow
balls; the elongated molecules line up side to side, like seeds on a dandelion head. In one
of the most influential origins publications in history, California biochemist David Deamer
described how he extracted a suite of these versatile organic molecules from the carbon-
richMurchisonmeteorite(aconglomerationofchemicalsformedindeepspacelongbefore
Earth) and found that they rapidly organized themselves into tiny cell-like spheres with
an inside and an outside, not unlike tiny oil drops in water. A few years ago Deamer and
I found that carbon-rich molecules forming under hot, pressurized black smoker condi-
tions behave in much the same way. These and other experiments reveal that membrane-
bound vesicles are an inevitable feature of the prebiotic world; lipid self-assembly must
have played a key role in life's origins.
