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into existence from non- life. For compartmentalization, we need to under-
20
stand how we got to bilayers made of phospholipids, and the answer might
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be found in fatty acids. For metabolism, we need to know how we got to
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cells driven by the proton- motive force, and the answer might be porous
23
chambers in alkaline vents. For replication, we need to know how we got to
24
DNA, and the answer might be RNA.
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The relationship of RNA to DNA is like the relationship of an oral tra-
26
dition of poetry to words written down in books. The same information
27
can be conveyed, but DNA is much more reliable and stable. Yet it is suffi-
28
ciently sophisticated that it’s hard to see how it could have come into exis-
29
tence by itself. When DNA gets copied, an important part of the work is
30
done by proteins. But the proteins are supposed to be constructed using
31
information encoded in the DNA. How could either one arise without the
32
other already being present?
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The favorite answer among abiogenesis researchers is a scenario called
34
RNA world. The basic idea was proposed by a number of people in the
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1960s, including Alexander Rich, Francis Crick, Leslie Orgel, and Carl
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Woese. DNA is good at storing information, and proteins are good at per-
03
forming biochemical functions; RNA is able to do both, although it’s not
04
as good at either one. RNA could have come along before either DNA or
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proteins, and served as the basis for a primitive and less robust form of early
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life, before evolution gradually distributed responsibilities to the more ef-
07
fective DNA and proteins.
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The role of RNA in extracting information from DNA was recognized
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fairly early on, but it wasn’t until later that biologists verified that RNA
10
could also act as a catalyst, expediting and governing the rate of biochemi-
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cal reactions. In particular, ribozymes, discovered in the 1980s, are a par-
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ticular kind of RNA that can catalyze their own synthesis, as well as that
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of proteins. The word “ribozyme” is annoyingly similar to “ribosome.” It
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turns out that the crucial part of the ribosome complex consists of ribo-
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zyme RNA. That is, the ribosome is mostly ribozyme. (It’s jargon like this
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that turns young scientists toward physics and astronomy.)
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Further investigations have shown that there are a number of different
18
types of RNA, responsible for a variety of functions inside the cell. In ad-
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dition to messenger RNA and ribosomal RNA, we also have transfer RNA
20
that brings amino acids to the right place to be made into proteins, regula-
21
tory RNA that helps guide the expression of genes, and more. These discov-
22
eries have helped popularize the RNA-world hypothesis. If you want to get
23
life started from a replication- first perspective, you need a molecule that can
24
carry genetic information without relying on other complex mechanisms
25
to reproduce itself. RNA seems to hit the sweet spot.
26
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27
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The idea that RNA may have been the first carrier of genetic information,
29
and was able both to self- reproduce and to assemble other biochemically
30
useful structures, is compelling and beautiful. Like any good paradigm, one
31
of the great features of the RNA world scenario is that it has spurred a
32
tremendous amount of exciting research.
33
Consider the fact that RNA can be an enzyme: it can catalyze chemical
34
reactions, both for self- assembly and for protein synthesis. Where did that
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ability come from? It’s pretty clear how a string of nucleotides can store
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information, but acting as an enzyme seems like a completely different kind
01
of talent.
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This question was addressed in an interesting experiment by David Bar-
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tel and Jack Szostak in 1993. (Szostak shared the Nobel Prize in 2009 for
04
his work on how chromosomes are protected when DNA divides.) Their
05
technique was basically a human- aided version of Darwinian evolution.
06
They started with a large amount of random RNA: trillions of molecules
07
with no particular sequence to their nucleotides. They then picked out a
08
fraction of those molecules, the ones that seemed to be associated with
09
somewhat higher rates of catalysis, and made many copies of those. This
10
procedure was repeated several times: look for RNA that seemed to be
11
catalyzing certain reactions, and make copies of it. At each copying stage,
12
random mutations occurred, which occasionally led to the copied RNA
13
being a better catalyst than its precursor. After just ten iterations of this
14
procedure, the results were clear: the last pool of molecules was approxi-
15
mately 3 million times better at catalyzing reactions than the original sam-
16
ple. It’s a vivid demonstration of how undirected, random mutation can
17
lead to enormous improvements in the ability of chemicals to perform bio-
18
logically useful functions.
19
Another exciting development came from biologists Tracey Lincoln and
20
Gerald Joyce in 2009. They were able to create a system of two RNA en-
21
zyme molecules— ribozymes— that together underwent self- sustained rep-
22
lication. Without any help from surrounding proteins or other biological
23
structures, these molecules are able to completely duplicate each other in
24
about an hour. Even better, the molecules occasionally mutate, and there-
25
fore undergo Darwinian evolution, with the more fit structures preferen-
26
tially surviving. It’s not a cell by any means, but you don’t need to strain to
27
see how it could be one of the steps along the road from chemistry to life.<
br />
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Even if RNA played a central role at the origin of life, we don’t yet have
29
a complete picture. Compartmentalization, metabolism, and replication all
30
have to be brought together. RNA and bilayers made from fatty acids may
31
be symbiotic— they could help each other flourish in the rough- and- tumble
32
environment of the early Earth. A membrane can shield the fragile RNA
33
from external commotion, helping it survive long enough to reproduce. An
34
RNA molecule, meanwhile, can attract other biological molecules into the
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membrane, helping it grow to the point where it will naturally split in
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two— a primitive form of cellular division.
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Fitting in metabolism may be trickier, though Szostak doesn’t think it’s
04
a big problem. He envisions a proto- cell, RNA encapsulated in a simple
05
membrane, floating in a pond that is warm on one end and cold on the
06
other. Convection currents push the proto- cell back and forth between the
07
two sides. In the cold end, the RNA grows by gathering nucleotides from
08
its surroundings, and two RNA strands huddle together as if seeking
09
warmth. When it drifts to the warmer side of the pond, the increased tem-
10
perature gradually peels the two strands apart; the membrane accretes a few
11
more fatty- acid molecules until it divides in two, and (hopefully, some-
12
times) we now have two proto- cells with a single strand of RNA each. They
13
both drift back to the cold side of the pond, and the cycle of proto- life be-
14
gins again.
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Russell and the metabolism- firsters don’t think it will be nearly that
16
easy. They believe that the hard part is assembling a complex system of
17
chemical reactions that can take advantage of the ambient free energy, set-
18
ting up proton- motive forces in chambers of porous underwater vents.
19
From there, they suggest, these reactions will naturally feed on any sur-
20
rounding free- energy fuel they can find. That might mean that they break
21
free of the rocks by entering fatty- acid membranes, and they keep going by
22
regulating their reactions through enzymes, which eventually be-
23
come RNA.
24
•
25
26
Or maybe both scenarios are right, or maybe neither is.
27
There is no reason to think that we won’t be able to figure out how life
28
started. No serious scientist working on the origin of life, even those who
29
are personally religious, points to some particular process and says, “Here
30
is the step where we need to invoke the presence of a nonphysical life- force,
31
or some element of supernatural intervention.” There is a strong conviction
32
that understanding abiogenesis is a matter of solving puzzles within the
33
known laws of nature, not calling for help from outside of them.
34
This conviction comes from the incredible historical track record sci-
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ence has established. While there are many questions about life’s origin that
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science hasn’t answered, there are a large number that it has, any one of
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which could have been a problem that science all by itself was unable to
01
address. (Recall Immanuel Kant’s confident proclamation that there will
02
never be a Newton for a blade of grass.) How do species evolve from earlier
03
species? How do organic molecules become synthesized? How do cellular
04
membranes assemble themselves? How can complex reaction networks
05
overcome free- energy barriers? How can RNA molecules develop the abil-
06
ity to act as catalysts for biochemical reactions? These are questions we have
07
answered. Our Bayesian credence that this string of successes will keep go-
08
ing should be very high indeed.
09
This perspective meets resistance in certain quarters, and not only
10
among religious fundamentalists. The idea that life could just start out of
11
no life at all isn’t obvious. We don’t see it taking place before our eyes, no
12
matter what Jan Baptist van Helmont might have imagined. Modern- day
13
organisms are mind- bogglingly complex, and made of individual parts that
14
work together amazingly well. The idea that it “just happened” is a chal-
15
lenging one.
16
Fred Hoyle, an esteemed British astrophysicist known for his staunch
17
opposition to the Big Bang model, attempted to quantify this unease. He
18
considered the configuration of atoms in a biological structure such as a
19
cell. Then, in a move taken from Ludwig Boltzmann’s playbook, he com-
20
pared the total number of ways such atoms could be arranged to the much
21
smaller number that would qualify as a cell. Multiplying together a bunch
22
of tiny numbers, he concluded that the chance of life assembling all by itself
23
is something like 1 in 1040,000.
24
Hoyle was a master of vivid imagery, and he illustrated his point with a
25
famous analogy:
26
27
The chance that higher life forms might have emerged in this
28
way is comparable to the chance that a tornado sweeping
29
through a junkyard might assemble a Boeing 747 from the ma-
30
terials therein.
31
32
The problem is that Hoyle’s version of “this way” is nothing at all like
33
how actual abiogenesis researchers believe that life came about. Nobody
34
thinks that the first cell occurred when a fixed collection of atoms was re-
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a cell- like configuration. What Hoyle is describing is essentially the
02
Boltzmann Brain scenario—
truly random fluctuations coming together to
03
create something complex and ordered.
04
The real world is different. The “unlikeliness” associated with low-
05
entropy configurations is built into the universe from the start, by the in-
06
credibly low entropy near the Big Bang. The fact that the development of
07
the cosmos proceeds from this very special initial condition, rather than
08
wandering through a more typical equilibrium ensemble of states, imposes
09
a strong nonrandom aspect on the evolution of the universe. The appear-
10
ance of cells and metabolism is a reflection of the universe’s progression
11
toward higher entropy, not an unlikely happenstance in an equilibrium
12
background. Like the swirls of cream mixing into coffee, the marvelous
13
complexity of biological organisms is a natural consequence of the arrow
14
of time.
15
We’ve made amazing progress in understanding what life is and how it
16
came to be, and there’s every reason to think that progress will continue
17
until we have figured it out. The work ahead will involve chemistry, physics,
18
mathematics, and biology, not magic.
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Evolution’s Bootstraps
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07
08
09
10
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12
I
13
n 1988, Richard Lenski had a brilliant idea: he was going to turn evo-
14
lutionary biology into an experimental science.
15
Evolution is the idea that provides the bridge from abiogenesis to
16
the grand pageant of life on Earth today. There’s no question that it’s a sci-
17
ence; evolutionary biologists formulate hypotheses, define likelihoods of
18
different outcomes under competing hypotheses, and collect data to update
19
our credences in those hypotheses. But chemists and physicists have an ad-
20
vantage over evolutionary biologists or, for that matter, astronomers: they
21
can perform repeated experiments in their labs. It would be very hard to set
22
up a laboratory experiment to see Darwinian evolution in action, just as it
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