1.3: Oceans and the Origins of Life

To investigate the origins of life, a number of laboratory studies have attempted to reproduce the conditions thought to exist in the early oceans and atmosphere, including high ultraviolet radiation levels and frequent lightning discharges. Under these conditions, a wide range of organic compounds are created from mixtures of carbon dioxide, methane, ammonia, hydrogen, and water. The organic compounds formed in these experiments included amino acids, which are considered the most important building blocks of the complex molecules needed for life to exist. However, we do not understand how or know where these building blocks were assembled into the much more complex molecules that were needed to form the first living matter. Several hypotheses have been proposed. The chemical reactions necessary to construct complex molecules may have taken place on the surface of solid particles, deep within the Earth, or at hydrothermal vents (Chap. 15). Another possible explanation for the origin of these complex compounds is that they first reached the Earth in meteorites.

All life as we know it depends on the transport of chemical elements and compounds within cells and between cells and the surrounding environment. In all known living organisms, from archaea to mammals, these chemical substances are transported dissolved in water. Therefore, water is essential to all life on Earth. As a result, it is not surprising that life appears to have been nurtured and developed in the oceans for most of the billions of years of Earth’s history. Figure 1-4 shows the 4.6 billion years of Earth’s history compared with a year with significant dates including the formation of the oceans, the first primitive animals, evolution of humans, and the 5 well-known mass extinctions.

Figure 1-4. The history of the Earth shown as if it were 1 year. The Earth formed about 4.6 billion years ago. Animals have existed on the Earth for only an extremely brief period of its history. If the age of the Earth were only 1 year, animals would have existed for only 2 months, our species Homo sapiens for about 9 minutes, and the science of oceanography for less than a single second. Most actual dates on the figure are expressed as millions of years ago (MYA) and are approximate.

Evidence from the fossil record indicates that life has existed on the Earth for a very long time. Although there is evidence that life began much earlier, the first life forms for which we have direct evidence were bacteria-like microorganisms that existed about 3600 million years ago (MYA), approximately 1 billion years after the Earth was formed (Fig. 1-4). When the oceans were first formed about 4200 MYA, the atmosphere consisted mostly of nitrogen, with smaller amounts of carbon dioxide and methane. Because there was no free oxygen in the atmosphere or oceans, it is thought that some of the earliest life forms were similar to the chemosynthetic microbes that are currently found in many isolated environments such as hydrothermal vents and deep within the Earth’s crust (CC14, Chap. 15). These environments contain no free oxygen and are often characterized by conditions of extreme temperature and pressure, similar to the conditions that are thought to have existed in the oceans and within Earth’s crust when the oceans were first formed.

If the first organisms did utilize chemical energy to support their energy needs, then at some unknown time the first species of microorganisms that utilized the sun’s light energy developed. These organisms were phototrophic (CC14). Early phototrophic microorganisms probably used hydrogen sulfide as their source of hydrogen, which is needed for building chemical compounds. Later, photosynthetic microorganisms, likely cyanobacteria, developed the ability to split water molecules, using the hydrogen to build their chemical compounds, releasing oxygen in the process. Over a period of time, between about 2000 and 1000 MYA, these photosynthetic organisms changed the composition of the Earth’s atmosphere. Oxygen concentrations steadily increased until they reached approximately the 20% present in the current atmosphere (Fig. 1-4). This change was fundamental to the development of the majority of living species now on the Earth. As the free oxygen accumulated in the atmosphere, it reacted with sulfides and other chemical compounds in the oceans that supported the life cycles of chemosynthetic species. As a result, chemosynthetic species largely disappeared or became restricted to extreme environments, such as those where they are found today. Furthermore, the free oxygen permitted the development of the animal kingdom. All species of animals depend on respiration using oxygen obtained from their environment.

The oceans and atmosphere reached a steady state (maintaining approximately the same chemical compositions that they have today) approximately 1000 MYA, and this relative stability permitted the development of more complex life forms. The first primitive higher animals were marine invertebrates, perhaps similar to sponges and jellies (jellyfish), that developed about 700 MYA, followed by the first fishes about 500 MYA (Fig. 1-4). The first plants appeared on land about 500 MYA, and the first mammals only about 220 MYA. Humans are latecomers. Hominids (humanlike species) have existed for only about 4 million years—less than one-tenth of 1% of the history of the Earth. In the 1-year model, this is less than 9 minutes (Fig. 1-4).

Although the specific environment in which life first developed is not known, the oceans clearly played a major role in nurturing the development of life on the Earth.