27-1 The chemical building blocks of life are found in space

Suppose you were the first visitor to a new and alien planet. If you saw a three-headed lizard running by, you would be sure it was an alien life-form. But how can you distinguish an alien microbe—or even just a fossil-like remnant of a microbe—from a dust grain? What might alien life look like? Questions such as these are central to astrobiology, the study of life in the universe. Most astrobiologists suspect that if we find living organisms on other worlds, they will be “life as we know it”—that is, their biochemistry will be based on the unique properties of the carbon atom, as is the case for all life on Earth.

Organic Molecules in the Universe

Figure 27-1: Complex Molecules and Carbon (a) Atoms that can bond to only two other atoms, like the atoms denoted X shown here, can form a chain of atoms called a linear molecule. The chain stops where we introduce an atom, such as those labeled Y and Z, that can bond to only one other atom. (b) A carbon atom (denoted C) can bond with up to four other atoms. Hence, carbon atoms can form more complex, nonlinear molecules like glucose. All organic molecules that are found in living organisms have backbones of carbon atoms.

Why carbon? The reason is that carbon has the most versatile chemistry of any element. Carbon atoms can form chemical bonds to create especially long and complex molecules (Figure 27-1). These carbon-based compounds, called organic molecules, include all the molecules of which living organisms are made. (Silicon has some chemical similarities to carbon, and it can also form complex molecules. But as Figure 27-2 shows, complex silicon molecules do not have the right properties to make up complex systems such as living organisms.)

Organic molecules can be linked together to form elaborate structures, such as chains, lattices, and fibers. Some of these structures are capable of complex, self-regulating chemical reactions. Furthermore, the primary constituents of organic molecules—carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus—are among the most abundant elements in the universe. The versatility and abundance of carbon suggest that extraterrestrial life is also likely to be based on organic chemistry.

If life is based on organic molecules, then these molecules must initially be present on a planet in order for life to arise from nonliving matter. We now understand that many carbon-based molecules originate from nonbiological processes in interstellar space. One such molecule is carbon monoxide (CO), which is made when a carbon atom and an oxygen atom collide and bond together. Carbon monoxide is found in abundance within giant interstellar clouds that lie along the spiral arms of our Milky Way Galaxy (see Figure 1-7) as well as in other galaxies (see Figure 1-9). Carbon atoms have also combined with other elements to produce an impressive array of interstellar organic molecules, including ethyl alcohol (CH3CH2OH), formaldehyde (H2CO), methylcyanoacetylene (CH3C3N), and acetaldehyde (CH3CHO). Radio astronomers have detected these molecules by looking for the telltale microwave emission lines of carbon-based chemicals in interstellar clouds.

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Figure 27-2: R I V U X G
Why Silicon Is Unsuitable for Making Living Organisms Like carbon, silicon atoms can bond with up to four other atoms. However, the resulting compounds are either too soft or too hard, or react too much or too little, to be suitable for use in living organisms. (a) Silicone has a backbone of silicon and oxygen atoms. The molecules form a gel or liquid rather than a solid, and react too slowly to undergo the rapid chemical changes required of molecules in organisms. (b) Molecules can also be made with a silicon-carbon-oxygen backbone, but the results (like this quartz crystal) are too rigid for use in organisms.
(a: Richard Megna/Fundamental Photographs; b: Ispace/Shutterstock)

The planets of our solar system formed out of interstellar material (see Section 8-5), and some of the organic molecules in that material must have ended up on the planets’ surfaces. Evidence for this comes from meteorites called carbonaceous chondrites, like the one shown in Figure 27-3. These meteorites are ancient, date from the formation of the solar system, and are often found to contain a variety of carbon-based molecules. The Murchison meteorite, which fell on Australia in 1969, contains more than 70 amino acids, and these organic molecules are some of the building blocks of life.

Figure 27-3: R I V U X G
A Carbonaceous Chondrite Carbonaceous chondrites are primitive meteorites that date back to the very beginning of the solar system. This sample is a piece of the Allende meteorite, a large carbonaceous chondrite that fell in Mexico in 1969. Chemical analyses of newly fallen specimens disclose that they are rich in organic molecules, many of which are the chemical building blocks of life.
(Detlev van Ravenswaay/Science Source)

The spectra of comets (see Section 7-5)—which are also among the oldest objects in the solar system—show that they, too, contain an assortment of organic compounds. In 2006, NASA’s Stardust mission returned samples from the atmosphere (or coma) of Comet Wild 2. The samples were very limited, but one type of amino acid was found. In 2014, a probe from the European Space Agency is scheduled to land on a comet’s surface for direct samples of its nucleus. Astrobiologists are hoping the lander finds a variety of amino acids, and the results will help them estimate how much of Earth’s organic building blocks came from comets.

Comets and meteoroids were much more numerous in the early solar system than they are today, and they were correspondingly more likely to collide with a planet. These collisions would have seeded the planets with organic compounds from the very beginning of our solar system’s history. Organic compounds are also found in interplanetary dust particles (see Section 15-5), which continually rain down on the planets. Once meteorites, comets, and interplanetary dust particles bring simple organic chemicals to a planet’s surface, additional chemical reactions can produce an even wider range of the complex organic compounds needed for life. Similar processes are thought to take place in other planetary systems, which are thought to form in basically the same way as did our own (see Figure 8-13 and the image that opens Chapter 8).

CONCEPT CHECK 27-1

Why would life-forms throughout the cosmos likely be based on carbon?

CONCEPT CHECK 27-2

How can organic molecules end up on the surfaces of planets?

The Miller-Urey Experiment

Comets and meteorites would not have been the only sources of organic material on the young planets of our solar system. In 1952, the American chemists Stanley Miller and Harold Urey demonstrated that under conditions that are thought to have prevailed on the young Earth, simple chemicals can combine to form the chemical building blocks of life. In a closed container, they prepared a sample of “atmosphere”: a mixture of hydrogen (H2), ammonia (NH3), methane (CH4), and water vapor (H2O), the most common molecules in the solar system. Miller and Urey then exposed this mixture of gases to an electric arc (to simulate atmospheric lightning) for a week. At the end of this period, the inside of the container had become coated with a reddish-brown substance rich in amino acids and other compounds essential to life.

Organic molecules do not have to come from living organisms—they can also be synthesized in nature from simple chemicals

Since Miller and Urey’s original experiment, most scientists have come to the conclusion that Earth’s primordial atmosphere was composed of carbon dioxide (CO2), nitrogen (N2), and water vapor outgassed from volcanoes, along with some hydrogen. Modern versions of the Miller-Urey experiment (Figure 27-4) using these common gases have also succeeded in synthesizing a wide variety of organic compounds. The combination of comets and meteorites falling from space and chemical synthesis in the atmosphere could have made the chemical building blocks of life available in substantial quantities on the young Earth.

Figure 27-4: An Updated Miller-Urey Experiment Modern versions of this classic experiment prove that numerous organic compounds important to life can be synthesized from gases that were present in Earth’s primordial atmosphere. This experiment supports the hypothesis that life on Earth arose as a result of ordinary chemical reactions.

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CAUTION!

It is important to emphasize that scientists have not created life in a test tube. While organic molecules may have been available on the ancient Earth, biologists have yet to figure out how these molecules gathered themselves into cells and developed systems for self-replication. Nevertheless, because so many chemical components of life are so easily synthesized under conditions that simulate the primordial Earth, it is reasonable to suppose that life could have originated as the result of chemical processes. Furthermore, because the molecules that combine to form these compounds are rather common, it seems equally reasonable that life could have originated in the same way on other planets.

Organic building blocks are commonplace throughout the universe, but their abundance does not guarantee that life is equally commonplace. If a planet’s environment is hostile, life may never get started or may quickly be extinguished. But we now have evidence that nearly Earth-sized planets orbit other stars (see Section 8-7) and that additional planetary systems are forming around young stars (see Section 8-4, especially Figure 8-8). It seems increasingly likely that Earthlike planets will be found orbiting other stars, and that conditions on some of these worlds may be suitable for life as we know it. The moons of some planets, in our solar system or associated with other stars, might also have environments suitable for life.

CONCEPT CHECK 27-3

What did Miller and Urey create when they passed electricity through their sample of “atmosphere” containing a mixture of hydrogen (H2), ammonia (NH3), methane (CH4), and water vapor (H2O)?