Unit Four. The Evolution and Diversity of Life
16. Prokaryotes: The First Single-Celled Creatures
16.2. How Cells Arose
It is one thing to make amino acids spontaneously and quite another to link them together into proteins. Recall from figure 3.6 that making a peptide bond involves producing a molecule of water as one of the products of the reaction. Because this chemical reaction is freely reversible, it should not occur spontaneously in water (an excess of water would push it in the opposite direction). Scientists now suspect that the first macromolecules to form were not proteins but RNA molecules. When “primed” with high-energy phosphate groups (available in many minerals), RNA nucleotides spontaneously form polynucleotide chains that might, folded up, have been capable of catalyzing the formation of the first proteins.
The First Cells
We don’t know how the first cells formed, but most scientists suspect they aggregated spontaneously. When complex carbon-containing macromolecules are present in water, they tend to gather together, sometimes forming aggregations big enough to see without a microscope. Try vigorously shaking a bottle of oil-and-vinegar salad dressing—tiny bubbles called microspheres form spontaneously, suspended in the vinegar. Similar microspheres might have represented the first step in the evolution of cellular organization. A bubble, such as those produced by soap solutions, is a hollow, spherical structure. Certain molecules, particularly those with hydrophobic regions, will spontaneously form bubbles in water. The structure of the bubble shields the hydrophobic regions of the molecules from contact with water. Such microspheres have many cell-like properties—their outer boundary resembles the membranes of a cell in that it has two layers (see pages 74 and 75), and the microspheres can increase in size and divide. Bubble models, such as the Lerman model discussed on the previous page, propose that over millions of years, those microspheres better able to incorporate molecules and energy would have tended to persist longer than others. Although it is true that lipid microspheres will form readily in water, there appears to be no hereditary mechanism to transfer these improvements from parent microsphere to offspring.
As we learned earlier, scientists suspect that the first macromolecules to form were RNA molecules, and with the recent discovery that RNA molecules can behave as enzymes, catalyzing their own assembly, this provides a possible early mechanism of inheritance. Perhaps the first cellular components were RNA molecules, and initial steps on the evolutionary journey led to increasingly complex and stable RNA molecules. Later, the stability of RNA might have improved even more when surrounded by a microsphere. Eventually DNA may have taken the place of RNA as the storage molecule for genetic information because the double-stranded DNA would have been more stable than single-stranded RNA.
When we speak of it having taken millions of years for a cell to develop, it is hard to believe there would be enough time for an organism as complicated as a human to develop. But in the scheme of things, human beings are recent additions. If we look at the development of living organisms as a 24-hour clock of biological time as shown in figure 16.3, with the formation of the earth 4.5 billion years ago being midnight, humans do not appear until the day is almost all over, only minutes before its end.
Figure 16.3. A clock of biological time.
A billion seconds ago, most students using this text had not yet been bom. A billion minutes ago, Jesus was alive and walking in Galilee. A billion hours ago, the first modern humans were beginning to appear. A billion days ago, the ancestors of humans were beginning to use tools. A billion months ago, the last dinosaurs had not yet been hatched. A billion years ago, no creature had ever walked on the surface of the earth.
As you can see, the scientific vision of life’s origin is at best a hazy outline. Although scientists have not disproven the hypothesis that life originated naturally and spontaneously, little is known about what actually happened. Many different scenarios seem possible, and some have solid support from experiments. Deep-sea hydrothermal vents are an interesting possibility; the prokaryotes populating these vents are among the most primitive of living organisms. Other researchers have proposed that life originated deep in the earth’s crust.
Because we know so little about how DNA, RNA, and hereditary mechanisms first developed, science is currently unable to resolve disputes concerning the origin of life. How life might have originated naturally and spontaneously remains a subject of intense interest, research, and discussion among scientists.
Key Learning Outcome 16.2. Little is known about how the first cells originated. Current hypotheses involve chemical evolution within bubbles, but this is an area of intense interest in research.
Has Life Evolved Elsewhere?
We should not overlook the possibility that life processes might have evolved in different ways on other planets. A functional genetic system, capable of accumulating and replicating changes and thus of adaptation and evolution, could theoretically evolve from molecules other than carbon, hydrogen, nitrogen, and oxygen in a different environment. Silicon, like carbon, needs four electrons to fill its outer energy level, and ammonia is even more polar than water.
Perhaps under radically different temperatures and pressures, these elements might form molecules as diverse and flexible as those carbon has formed on earth.
The universe has 1020 (100,000,000,000,000,000,000) stars similar to our sun. We don't know how many of these stars have planets, but it seems increasingly likely that many do. Since 1996, astronomers have been detecting planets orbiting distant stars. At least 10% of stars are thought to have planetary systems. If only 1 in 10,000 of these planets is the right size and at the right distance from its star to duplicate the conditions in which life originated on earth, the "life experiment” will have been repeated 1015 times (that is, a million billion times). It does not seem likely that we are alone.
A dull gray chunk of rock collected in 1984 in Antarctica ignited an uproar about ancient life on Mars with the report that the rock contains evidence of possible life. Analysis of gases trapped within small pockets of the rock indicate it is a meteorite from Mars. It is, in fact, the oldest rock known to science—fully 4.5 billion years old. Evidence collected by the 2004 NASA Mars mission (the photo below of the Martian surface was taken by the rover, Spirit) suggests that the surface, now cold and arid, was much warmer when the Antarctic meteorite formed 4.5 billion years ago, that water flowed over its surface, and that it had a carbon dioxide atmosphere—conditions not too different from those that spawned life on earth.
When examined with powerful electron microscopes, carbonate patches within the meteorite exhibit what look like microfossils, some 20 to 100 nanometers in length. One hundred times smaller than any known bacteria, it is not clear they actually are fossils, but the resemblance to bacteria is striking.
Viewed as a whole, the evidence of bacterial life associated with the Mars meteorite is not compelling. Clearly, more painstaking research remains to be done before the discovery can claim a scientific consensus. However, while there is no conclusive evidence of bacterial life associated with this meteorite, it seems very possible that life has evolved on other worlds in addition to our own.
There are planets other than ancient Mars with conditions not unlike those on earth. Europa, a large moon of Jupiter, is a promising candidate (photo above). Europa is covered with ice, and photos taken in close orbit in the winter of 1998 reveal seas of liquid water beneath a thin skin of ice. Additional satellite photos taken in 1999 suggest that a few miles under the ice lies a liquid ocean of water larger than earth's, warmed by the push and pull of the gravitational attraction of Jupiter's many large satellite moons. The conditions on Europa now are far less hostile to life than the conditions that existed in the oceans of the primitive earth. In coming decades, satellite missions are scheduled to explore this ocean for life.