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useful might be a clue that it took advantage of it right from the start.
29
This is where the hydrogenation of carbon dioxide comes in. Russell’s
30
comment alludes to the fact that there is free energy locked up in a mixture
31
of carbon dioxide (CO ) and hydrogen gas (H ), both of which were abun-
32
2
2
dant in certain environments on the young Earth. If the carbon could
33
somehow shed its two oxygen atoms and replace them with hydrogen, we
34
could end up with methane (CH ) and water (H O). That’s a configuration
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that has less free energy; as far as the second law of thermodynamics is
02
concerned, it’s a transformation that “wants” to happen.
03
It doesn’t happen all by itself. Anytime you light a candle, or set any-
04
thing else on fire, you are releasing free energy by combining the fuel with
05
oxygen. But candles don’t just burst into flame; they require a spark to start
06
the reaction.
07
In the case of carbon dioxide, we require something more elaborate than
08
a spark. It’s easy to invent sequences of reactions that gradually move the
09
oxygens off of the carbon atom and replace them with hydrogen. The prob-
10
lem is that, while the sequence as a whole releases energy, the first required
11
steps actually cost energy, and therefore don’t happen by themselves. Extract-
12
ing the free energy from carbon dioxide is like robbing a bank: there’s a lot
13
of money in there, but you have to go to a great deal of effort to get it out.
14
A number of researchers, including William Martin and Nick Lane as
15
well as Russell, have been working hard on exploring scenarios in which the
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right sequence of reactions could have come together in just the right way
17
to take advantage of the ambient free- energy bounty. They have a couple of
18
tricks at their disposal. One is catalysis: hastening along the reaction you
19
want by taking advantage of nearby compounds that aren’t themselves re-
20
acting but can change the shape or properties of the chemicals that are in-
21
volved. Another is disequilibrium: an imbalance in conditions at nearby
22
locations that can be used to drive the desired reactions.
23
These ingredients come together in the right way in a specific environ-
24
ment: deep- sea hydrothermal vents. In particular, alkaline vents— ones
25
where proton- attracting alkaline chemicals are produced. They’re not the
26
only plausible environment in which we can search for life’s origin; as just
27
one other example, serpentine mud volcanoes are another ocean- floor
28
structure that might be hospitable to early life. But alkaline vents have some
29
nice properties.
30
As early as 1988, Russell predicted, on the basis of his vision for life’s
31
origin, a particular kind of underwater geological formation that had not
32
yet been discovered: underwater vents that were alkaline, warm (but not
33
too hot), highly porous (riddled with tiny pockets, like a sponge), and rela-
34
tively stable and long- lasting. The idea was that the pockets could provide a
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kind of compartmentalization even before the existence of any kind of
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organic cell membranes, and the disequilibrium between alkaline chemi-
01
cals in the vents and the proton- rich acidic ocean water all around would
02
naturally produce a version of the proton- motive force so beloved by bio-
03
logical cells.
04
In 2000, Gretchen Früh- Green, on a ship in the mid- Atlantic Ocean as
05
part of an expedition led by marine geologist Deborah Kelley, stumbled
06
across a collection of ghostly white towers in the video feed from a robotic
07
camera near the ocean floor deep below. Fortunately they had with them a
08
submersible vessel named Alvin, and Kelley set out to explore the structure
09
up close. Further investigation showed that it was just the kind of alkaline
10
vent formation that Russell had anticipated. Two thousand miles east of
11
South Carolina, not far from the Mid- Atlantic Ridge, the Lost City hydro-
12
thermal vent field is at least 30,000 years old, and may be just the first
13
known example of a very common type of geological formation. There’s a
14
lot we don’t know about the ocean floor.
15
The chemistry in vents like those at Lost City is rich, and driven by the
16
sort of gradients that could reasonably prefigure life’s metabolic pathways.
17
Reactions familiar from laboratory experiments have been able to produce
18
a number of amino acids, sugars, and other compounds that are needed to
19
ultimately assemble RNA. In the minds of the metabolism- first contingent,
20
the power source provided by disequilibria must come first; the chemistry
21
leading to life will eventually piggyback upon it.
22
Albert Szent- Györgyi, a Hungarian physiologist who won the Nobel
23
Prize in 1937 for the discovery of vitamin C, once offered the opinion that
24
“life is nothing but an electron looking for a place to rest.” That’s a good
25
summary of the metabolism- first view. There is free energy locked up in
26
certain chemical configurations, and life is one way it can be released. One
27
compelling aspect of the picture is that it’s not simply working backward
28
from “We know there’s life; how did it start?” Instead, it’s suggesting that
29
life is the solution to a problem: “We have some free energy; how do we
30
liberate it?”
31
Planetary scientists have speculated that hydrothermal vents, similar to
32
Lost City, might be abundant on Jupiter’s moon Europa or Saturn’s moon
33
Enceladus. Future exploration of the solar system might be able to put this
34
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•
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In the ecosystem of abiogenesis researchers, metabolism- first proponents
04
are a plucky minority. The most popular approach, as mentioned earlier, is
05
replication- first.
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Metabolism is essentially “burning fuel,” something we see all around
07
us, from lighting a candle to starting a car engine. Replication seems harder,
08
more precious, difficult to obtain. If there is any part of “life” that might
09
act as a bottleneck to getting it started, it’s the fact that living beings repro-
10
duce themselves.
11
Fire is a well- known chemical reaction that readily reproduces itself,
12
leaping from tree to tree in a forest, but by most definitions it doesn’t count
13
as alive. We want something that carries information through the reproduc-
14
tion process: something whose “offspring” keep some knowledge of where
15
they came from.
16
There’s a simple example of such a thing: crystals. Certain kinds of at-
17
oms can organize themselves into regular patterns, which are then called
18
crystals. The same atoms might support different possible crystalline struc-
19
tures: when carbon arranges itself in a cubic pattern, we get diamond, but
20
if it’s in a hexagonal pattern, all we have is graphite. Crystals can grow by
21
adding new atoms, and can then divide by the simple expedient of breaking
22
in two. Each of the offspring will have inherited the structure of its parent
23
crystal.
24
That’s still not life, though we’re getting closer. While the basic crystal-
25
line structure can be inherited, variations in that structure— random
26
mutations— cannot. Variations are certainly possible; real crystals are often
27
riddled with impurities, or suffer from defects where the structure doesn’t
28
follow the dominant pattern. But there’s no way to pass down knowl-
29
edge of these variations to subsequent generations. What we want is a con-
30
figuration that is crystal- like (in that there is a fixed structure that can be
31
reproduced) but more elaborate than a simple repeating pattern.
32
The kind of thing we need was described by John von Neumann, a bril-
33
liant Hungarian American mathematician who played crucial roles in the
34
development of quantum mechanics, statistical mechanics, and game the-
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ory. In the 1940s, he laid out in abstract terms what would be required for
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a system to reproduce itself and evolve in an open- ended way. His (purely
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t h E OR I g I n A n d P u R P O S E O F l I F E
mathematical) machine— the “von Neumann Universal Constructor”—
01
included not only a mechanism for actually performing the self- replication,
02
but also a “tape” that encoded the structure of the machine. Von Neumann–
03
like self- replicators have been implemented in computer simulations, com-
04
plete with the possibility of mutation and evolution. No one has yet built a
05
large- scale physical machine that would behave this way, but there’s noth-
06
ing in the laws of physics that would prevent us from doing so, and NASA
07
and other organizations have seriously investigated the possibility. Would
08
a physical implementation of a von Neumann Universal Constructor qual-
09
ify as “alive”?
10
11
•
12
Erwin Schrödinger, in What Is Life? , recognized the need for information
13
to be passed down to future generations. Crystals don’t do the job, but they
14
come close; with that in mind, Schrödinger suggested that the culprit
15
should be some sort of “aperiodic crystal”— a collection of atoms that fit
16
together in a reproducible way, but one that had the capacity for carrying
17
substantial amounts of information, rather than simply repeating a rote
18
pattern. This idea struck the imaginations of two young scientists who went
19
on to identify the structure of the molecule that actually does carry genetic
20
information: Francis Crick and James Watson, who deduced the double-
21
helix form of DNA.
22
Deoxyribonucleic acid, DNA, is the molecule that essentially all known
23
living organisms use to store the genetic information that guides their func-
24
tioning. (There are some viruses based on RNA rather than DNA, but
25
whether or not they are “living organisms” is disputable.) That information
26
is encoded in a series of just four letters, each corresponding to a particular
27
molecule called a nucleotide: adenine (A), thymine (T), cytosine (C), and
28
guanine (G). These nucleotides are the alphabet in which the language of
29
genes is written. The four letters string together to form long strands, and
30
each DNA molecule consists of two such strands, wrapped around each
31
other in the form of a double helix. Each strand contains the same informa-
32
tion, as the nucleotides in one strand are paired up with complementary
33
ones in the other: A’s are paired with T’s, and C’s are paired with G’s. As
34
Watson and Crick put it in their paper, with a measure of satisfied under-
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postulated immediately suggests a possible copying mechanism for the ge-
02
netic material.”
03
In case it has managed to escape your notice, the copying mechanism is
04
this: the two strands of DNA can unzip from each other, then act as tem-
05
plates, with free nucleotides fitting into the appropriate places on each
06
separate strand. Since each nucleotide will match only with its specific kind
07
of partner, th
e result will be two copies of the original double helix— at
08
least as long as the duplication is done without error.
09
The information encoded in DNA directs biological operations in the
10
cell. If we think of DNA as a set of blueprints, we might guess that some
11
molecular analogue of a construction worker comes over and reads the blue-
12
prints, and then goes away to do whatever task is called for. That’s almost
13
right, with proteins playing the role of the construction workers. But cel-
14
lular biology inserts another layer of bureaucracy into the operation. Pro-
15
teins don’t interact with DNA directly; that job belongs to RNA.
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Protein
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DNA
RNA
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RNA is similar in structure to DNA, but it usually comes in the form
01
of single strands. The “backbone” of the strands differs slightly from RNA
02
to DNA, and RNA pairs adenine with a nucleotide called uracil (U), rather
03
than with thymine. It’s less chemically stable than DNA, but it can carry
04
equivalent information in its particular sequence of nucleotides.
05
Information gets out of DNA when the two strands unzip, and their
06
sequences are copied onto RNA segments. Those segments, called messen-
07
ger RNA, carry genetic information to a different unit within the cell,
08
the ribosome. Ribosomes, discovered back in the 1950s, are complicated
09
structures that take the information in the RNA and use it to construct
10
proteins. This multistep process enables a relatively stable information-
11
storage system (DNA) to construct useful molecules (proteins) using less
12
stable messengers (RNA) and a complete separate construction facility (the
13
ribosome).
14
15
•
16
Just as for compartmentalization and metabolism, replication faces a “How
17
did we get here from there?” problem, relating the sophisticated structures
18
of modern- day biology to simpler systems that could plausibly have come
The Big Picture Page 45