Key Innovations Open New Realms

Occasionally during the history of life, the process of evolution by natural selection has produced characteristics that radically change the future course of evolution. These characters are called "key innovations", because they open up possibilities for life and the evolution of characters that were simply not possible before they evolved. One of the biggest breakthroughs in the history of life occurred 3 billion years ago. This was the evolution of pigments in bacteria that could accomplish photosynthesis, that is the capture of light energy and the production of sugars from water and carbon dioxide.

Life's first cells subsisted by anaerobially (i.e., without oxygen) metabolizing inorganic compounds. This type of metabolism is not very efficient, and provides relatively little energy. Thus, the pace of early life was slow, and food supplies were limited to available (and non-renewable) inorganic molecules. The evolution of photosynthetic pigments within cyanobacteria that capture the energy of the sun and permit the construction of sugars, a renewable food source, was literally an Earth-changing event for several reasons.

First, oxygen is released as a waste product from the process of photosynthesis. Up until this time in the history of life, all life was anaerobic, and oxygen is poisonous to these cells. However, oxygen was produced by the first photosynthetic bacteria, free oxygen was not liberated because iron in the Earth's crust took it all up, forming iron oxide. This process (the "rusting" of the world) lasted for 1.2 billion years, until the iron stocks were saturated. Then, free oxygen began accumulating in the atmosphere as O₂. This free oxygen relegated anaerobic bacteria forever to a minor role, as most of Earth's habitats became saturated with oxygen.

Secondly, for the first time on Earth, new food was made by the organisms themselves. Previously, all nutrition was obtained from existing inorganic molecules. The production of a renewable resource from the energy of the sun and commonly available molecules permitted a huge expansion in the extent and diversity of life on Earth.

The evolution of photosynthesis and the resulting change of the atmosphere from anaerobic to aerobic, along with the evolution of new food sources was an irreversible change in the environment on Earth, one which would be part of the landscape for all of the major phyla of organisms (including all multicellular organisms) that evolved after that time. Photosynthesis initiated the inextricable interdependence of plants and animals. Plants are the only organisms that can harness the energy of the sun and use it to make food. The waste of plants (oxygen) is a requirement for animals, while the waste gas from animal respiration (carbon dioxide) is the raw material needed for photosynthesis. The required balance of plant and animal life on Earth was in place from that point onward. If photosynthesis or its equivalent had evolved using a different chemical mechanism, which seems possible under different conditions, the entire history of life would have been different. Is there any reason to believe that Earth-like photosynthesis would evolve on other worlds? It seems unlikely.

Following the evolution of photosynthesis, chance variation in biochemical pathways led to further key innovations. Some lineages of prokaryotes were able to utilize the waste oxygen for aerobic metabolism. Metabolism using oxygen is much more efficient than is anaerobic metabolism and liberates a great deal more energy, permitting the evolution of increasingly active life styles, including the evolution of the first predators, which were single celled prokaryotes. Some of these prokaryotic predators ingested aerobically metabolizing bacteria. By chance, some of these prey were not digested, but came to live symbiotically within the "predator" cell. Eventually (about 2-1.1 billion years ago), these evolved into mitochondria, the powerhouses of the eukaryotic cell. A similar ingestion of a cyanobacterium in a lineage of protists leading to plants is thought to have led to the evolution of the chloroplast. With these two events, eukaryotic cells gained an enormous ability to make and process energy. The evolution of cellular organelles paved the way for the eventual evolution of plants and animals. This was a huge breakthrough in the history of life, without which the enormously active animal phyla could not have evolved.

So, long before organisms became multicellular, chance events had altered the trajectory of life - a particular genetic code was found and put into use by self-replicating nucleic acids, photosynthesis and aerobic metabolism evolved, and cells took on organelles (mitochondria and chloroplasts) that made new levels of activity possible. Would this same trajectory happen again on our Earth? Would the evolution of life on other worlds even be close?

After the evolution of multicellularity, about 1 billion years ago, natural selection within different lineages of single celled Eukaryotes led to the evolution of plants, fungi and animals, the three main eukaryotic Kingdoms. Within animals, the evolution of the invertebrate phyla began, and during the "Cambrian explosion" between 544 - 505 million years ago, all of the animal phyla present on earth today (and evidently a great many more) appeared. Over time, various key innovations evolved from chance variation arising in the genetic code, which then spread through populations by the advantages that the variant characteristics provided to organisms that bore them. These included the evolution of tissues, bilateral symmetry (which permitted the evolution of a head end and the accompanying concentration of sensory organs there), the elaboration of specialized organ systems, the evolution of the exoskeleton (in insects, spiders and aquatic arthropods like crabs), and, eventually, the evolution of a vertebral column (see figure 2, which shows the major animal phyla and the key innovations that spurred their evolution).

Even after the first evolutionary appearance of the vertebrates, there was still a considerable period of evolution required before the evolution and diversification of the mammals, the primates, and finally humans. Major innovations made the colonization of land and the evolution of mammals possible, including impermeable skin, the evolution of internal fertilization, the evolution of placental development of offspring, and nourishment through milk, and the evolution of homeostasis, which permits mammals to hold a constant body temperature. These characteristics produced lineages that provided great amounts of parental care to developing juveniles, permitting skills to be passed from parent to offspring. The tremendous biological diversity of the world into which mammals evolved may itself have favored the evolution of larger brain size through natural selection. No doubt other characteristics of the environment in which hominids evolved favored the evolution of large and active brains. Would these circumstances inevitably arise again during the course of a different bout of evolution?

Does the process of evolution itself necessarily lead to complexity, regardless of the details of exactly the forms of life that are evolving? It seems true that the very process of biological evolution is an elaborative one in which diversity of forms necessarily would be expected to increase. With that diversity comes new environments, new ecological opportunities, more biological possibilities. Would this always lead to complexity in the forms of life? In particular, would always be expected to lead to intelligence?

The final kind of contingency that has played a significant role in the course of evolution of life on Earth has been periodic extinction. Though there have been 6 periods in the history of life severe enough to be termed "mass extinctions", many more minor periods of large scale extinction have been noted. Mass extinctions have generally been caused by large-scale physical changes on the Earth, such as the fusing of the continents into a single land mass called Pangaea, or its subsequent breaking apart, though minor extinctions have been caused by a range of events, including activities of humans. Overhunting is thought to have caused the extinction of large mammals in North America about 10,000 years ago, and by all accounts, we are currently in the midst of a mass extinction of human origins (see Wilson, 1992).

During each mass extinction, the majority of life forms present at the time have gone extinct. This wholesale elimination of types of organisms not only extinguishes those lineages, but opens new opportunities for the lineages that manage to survive. In fact, each of the mass extinctions has heralded a new age in the history of life. For example, reptiles ruled the Earth for millions of years, but after the mass extinction 65 million years ago, the Age of Mammals dawned. During the time of the dinosaurs, small mammals had existed, but after the huge reptiles went extinct, mammals proliferated and diversified within a few million years. Ecological opportunities that had been dominated by reptiles became the purview of mammals. If the dinosaurs had not gone extinct, would humans have evolved? If we are currently involved in a mass extinction, what will be the next group to dominate the Earth?

The time at which a mass extinction occurs also plays a role in the subsequent trajectory of life's evolution. For example, when the dinosaurs went extinct, mammals were already present in low numbers and diversity. Thus, variants within mammal lineages were present and able to spread into the habitats dominated by dinosaurs through natural selection. If mammals had not yet evolved by 65 million years ago, those habitats might have been recolonized by a rediversification of reptiles or some other lineage. In an earlier extinction (about 440 million years ago), many marine invertebrates went extinct, opening the door for the diversification of fishes, leading to the Age of Fishes. Mammals could not have diversified and come to dominance then because the mammalian form had not yet evolved. The results of evolution after an extinction depend upon when it occurs.

Contributed by: Dr. Sara Via