The Origins of Life

The Origins of Life

Lecture 15

And it’s now very, very clear—it’s been repeated using different gases— that creating these simple chemicals is really not a huge problem in an environment without free oxygen.

Even the simplest living organisms are extremely complicated, so explaining how the rst organisms were created is a tough challenge. Fred Hoyle (1915–2001), one of the pioneers of modern cosmology, put it like this:

A junkyard contains all the bits and pieces of a Boeing-747, dismembered and in disarray. A whirlwind happens to blow through the yard. What is the chance that after its passage a fully assembled 747, ready to y, will be found standing there? (Hoyle, The Intelligent Universe, p. 19)

Hoyle made this comparison because both Boeing 747s and yeast cells contain about 6 million parts. Hoyle’s solution to the puzzle of life’s origins was to argue that life must have evolved somewhere else in the Universe before arriving on Earth. Yet most modern biologists are convinced that life did evolve on Earth, and that the idea of natural selection provides part of the explanation. This lecture summarizes some of the main ideas of modern explanations of the origins of life on Earth.

Traditional explanations for the origins of life can be divided into the divine and the naturalistic. Divine explanations suppose that life was created by a divine being. As we’ve seen before, modern science excludes such theories because they beg further questions (who created the creator?) and they cannot be tested scienti cally. It does not assert that such theories are wrong, but merely that they cannot be tested scienti cally. Naturalistic explanations suppose that life can be generated spontaneously from existing materials and forces. The Greek philosopher Aristotle (384–322 B.C.E.) saw the appearance of maggots in rotting meat as an example of spontaneous generation.

During the Scienti c Revolution, scientists began to test such ideas more rigorously. In 1765, Lazzaro Spallanzani (1729–1799) claimed to have refuted Aristotle’s idea by showing that if a meat broth was sterilized by boiling and then placed in an airtight container no microorganisms appeared. Opponents argued that there might be a “life force” in the air that “animated” living things, Though we don’t yet and Spallanzani’s containers had merely know all the details, we excluded that life force. understand enough to know In 1862, Louis Pasteur (1822–1895) that life can be assembled refuted the notion of a life force in from nonliving ingredients. a remarkably simple and elegant experiment. He boiled a broth in a ask with a long swan-necked outlet, open to the air. He argued that if there was a life force, it could enter, while seeds or spores would get trapped in the bend. His retorts can still be seen today in the Pasteur Institute in Paris, and they remain sterile. Pasteur’s experiment seemed to prove that life could only come from previous life forms, from eggs or spores. Spontaneous generation was impossible. If so, how were the rst living organisms created? Biologists began to get a grip on this knotty problem early in the 20th century. Modern approaches explain the origins of life in distinct stages. First, we must explain the creation of the simple molecules present in all living organisms: the amino acids that make proteins, the nucleic acids that make DNA, the carbohydrates that make sugars and starches, and the lipids that make fats and hormones. Today, atmospheric oxygen destroys such molecules, which is why, as Pasteur claimed, life can no longer be generated spontaneously. However, in the 1920s, Alexander Oparin in Russia and J. B. S. Haldane in Britain pointed out that such molecules could have thrived in an oxygen-free atmosphere, such as that of the early Earth. How could you test such an idea? In 1952, a graduate student, Stanley Miller, lled a glass tube with gases such as methane, ammonia, and hydrogen that might have been present in the early atmosphere, while carefully excluding oxygen. He added water, because complex chemical reactions are much easier in liquids than in gases

(where atoms are usually too far apart to react) or in solids (where atoms are locked so tightly together that there can be little change). He added energy in the form of heat and electric charges. Within days a dark red sludge appeared containing amino acids, nucleotides, and phospholipids. Later versions of the experiment, using different gases, have shown that all life’s basic chemicals could have formed spontaneously on the early Earth.

Second, we must explain the evolution of the much larger and more complex molecules found in living cells. The Urey-Miller experiment generated simple molecules with just a few atoms. Yet even the simplest viruses contain billions of atoms in complex con gurations, many arranged in huge chains. How could such huge and complex molecules have formed? As Fred Hoyle argued, it was unlikely that such molecules would be assembled by pure chance. Yet there is an answer to Hoyle’s riddle, and it involves natural selection. Though random changes are unlikely to create living organisms, if each successful step toward life can be locked into place, then the odds improve drastically. This is precisely how natural selection works, by locking into place random variations that create viable life forms. The idea that chemicals can “evolve” through a chemical version of natural selection (“chemical evolution”) underlies modern theories of the origins of life.

In the 1950s and 1960s, Sydney Fox (1912–) showed how “chemical evolution” might work. Under certain conditions, organic molecules spontaneously form long chains similar to those in living organisms. Some of these molecules naturally curl up to form cell-like spheres with semipermeable membranes through which they can ingest chemicals from outside (eating?). They can also divide (reproduction?). With a metabolism and the ability to reproduce, they can also adapt over time, giving them all the “emergent properties” of life. Where might such reactions have occurred in the early Earth? Darwin assumed they might have occurred in a “warm pond,” perhaps on the edge of the seas. Yet early in the Earth’s history, its surface would have been extremely dangerous, so today it seems more likely that life evolved under the seas, near mid-oceanic vents. Here there was energy, a rich mix of organic chemicals, and protection from ultraviolet radiation. Today, rich colonies of chemical-eating bacteria thrive in such environments.

Third, we must explain the creation of the exquisitely organized billionatom molecules of DNA (deoxyribonucleic acid), the “software” that controls reproduction. This is the toughest part of the puzzle. Without DNA, reproduction was inaccurate, and “chemical evolution” would have been slow and unreliable. DNA consists of two linked chains of nucleotides, linked by bonds like rungs on a ladder. Each bond consists of two “bases” (small clusters of atoms) that come in only four types and can only t together in certain ways. They are known as Adenine, Thymine, Cytosine, and Guanine. A (Adenine) links only with T (Thymine), and C only with G. So, along each chain, you have a sequence of the four bases, each linked to its complement on the other chain. The exact sequence of these bonds codes the information used to construct each organism. When DNA reproduces, the bonds split in two and the two chains separate. The bases seek out their complements from the chemicals surrounding them (A looks for T and so on), and in this way two new chains of DNA appear, identical to the originals.

This mechanism is the key to accurate reproduction. But there’s a problem. DNA cannot exist on its own, yet cells cannot survive without DNA—so which evolved rst, the software of DNA or the hardware of the cell? At present, the best bet is that RNA (ribonucleic acid), a single-stranded variant of DNA, may have acted as both “hardware” and “software.” Because it is similar to DNA, RNA can code for genetic information, but it can also act like an enzyme and help manufacture the molecules that cells need. However it was done, life seems to have appeared quickly on Earth, for living organisms existed by 3.8 billion years ago. This suggests that life may appear everywhere in the Universe that conditions are right. Though we don’t yet know all the details, we understand enough to know that life can be assembled from nonliving ingredients.

Essential Reading

Christian, Maps of Time, chap. 4. Delsemme, Our Cosmic Origins, chap. 5.

Supplementary Reading

Questions to Consider

Cairns-Smith. Seven Clues to the Origin of Life. Davies, The Fifth Miracle. Dyson, Origins of Life.

1. Why did the arguments of Haldane and Oparin shift discussions of the origin of life in new directions?

2. How satisfactory are modern naturalistic explanations of the origins of life?