The oldest fossils that we could call human are from the genus Australopithecus (“southern ape”), found in sediments in what is now Ethiopia. About three and a half million years old, these fossils are of a creature who walked upright, was about three feet tall, and had a brain the size of a modern newborn baby. There are well over a dozen different types of Australopithecus and early Homo that lived in Africa between three and one million years ago, and deciding which of these are in the actual line of descent of modern humans is still a matter that is intensely debated. In any case, individuals of what are called anatomically modern humans, Homo sapiens, appeared on the scene in Africa about 200,000 years ago. A close cousin, Homo neanderthalensis, or Neanderthal man, lived in Europe and the Middle East from about this time to roughly 35,000 years ago, when it went extinct. Thus, although there have been many close relatives of modern humans in the past, there are none alive today.
The Rate of Evolution
Most of today’s evolutionary scientists focus not on whether evolution occurred, but how it occurred. When Darwin first proposed his theory, he argued that evolution proceeds at a slow, steady rate, and that small changes gradually accumulate to produce large ones. This view is known today as gradualism. In the early 1970s, two American paleontologists—Stephen Jay Gould and Niles Eldredge—proposed an alternative theory that goes under the name of punctuated equilibrium. In their view, evolution is characterized by long periods of little change, interspersed (punctuated) by short periods of rapid change.
The fossil record simply isn’t good enough to allow us to differentiate between these two competing theories. Take trilobites, one of the fanciest fossils of the Paleozoic, which are proving especially well suited to studies of evolutionary rates. Each individual trilobite has a well-defined number of segments, but different species have different numbers. Trilobite experts count segments for many trilobites from different times and identify systematic changes. In some rock sequences changes seem to be sudden, but other deposits reveal more gradual shifts. It is likely that evolution proceeds in both gradual and punctuated ways under different circumstances, but scientists will need more fossil data before they can resolve the issue.
EXTINCTION
The fossil record is unambiguous: life on Earth has evolved from one-celled microorganisms to simple soft-bodied animals and plants to the remarkable diversity of forms and functions we see today. Countless millions of species have come into being, and almost as many have become extinct. Extinction is as much a part of evolution as the appearance of new forms.
The average lifetime of a species in the fossil record is a few million years. Given the fact that a substantial fossil record extends 542 million years into the past, you can see that the 10 million or so species that inhabit our planet today are a small group compared to those that have lived and disappeared in the past.
Trilobites, among the most distinctive of Paleozoic fossils, have complex forms that are ideal for studying small evolutionary changes. Some paleontologists spend years collecting trilobites and counting the exact number of body segments or eye facets to document these changes.
Mass Extinctions
Darwin and his contemporaries viewed evolution, as well as its inevitable companion extinction, as ongoing properties of life. They thought that species appear and disappear at a relatively constant rate. The fossil record argues against this view.
Life on Earth has suffered numerous mass extinctions, the most notable being the one that wiped out the dinosaurs about 66 million years ago. Although we are all familiar with the dinosaurs, two facts about this extinction are less well known than they should be. They are: (1) at the same time the dinosaurs were dying, fully two-thirds of all other species on Earth were being wiped out in what paleontologists call a mass extinction, and (2) this was neither the most intense nor the most recent mass extinction in the fossil record. At the end of the Paleozoic, 251 million years ago, fully 90 percent of existing species became extinct, while another event a mere 11 million years ago had a death toll of approximately 30 percent.
One intriguing idea about the cause of mass extinctions, and one backed by an increasing amount of data, is that they result from the impact of large asteroids or comets. Such an impact would send an immense, sun-blocking cloud of dust into the upper atmosphere. Months of sunless days would be enough to lower temperatures, deplete food supplies, and kill off most life on Earth.
ID: A New Challenge to Evolution
Ever since Darwin first proposed his theory of evolution by natural selection the subject has been a lightning rod for objections. The latest round of opposition, and the focus of efforts to stop teaching evolution in the public schools, is in the guise of the doctrine of intelligent design, or ID. Proponents of ID claim that life on Earth is so unimaginably intricate and complex that it could not possibly have emerged through any natural process. Rather, a super-intelligent engineer must have done the job (though most ID advocates do not talk about who designed the designers). Unlike young-Earth creationism, ID does make a few testable predictions, albeit negative ones. They predict, for example, that the complex tail-like flagellum that powers many microbes could not have arisen by a natural sequential process. If scientists succeed in demonstrating that it is possible in principle to detail an evolutionary process by which a flagellum arose (and a number of microbiologists are working on that problem), the claims of ID will be shown to be false. The problem with ID proponents is that they have not made a single testable prediction that has been shown to be true. Evolution, by contrast, has made countless thousands of confirmed predictions regarding the continuous, progressive tree of life.
In 2006, the issue of whether ID should be taught in the public schools as an alternative to evolution was brought into federal court in Dover, Pennsylvania. Members of the Dover school board had ordered teachers to read a statement about ID in their science classrooms, but the teachers refused on the ground that it violated their contractual obligation to teach the best science. The members of the board then came into the school and read the statement themselves, at which point the parents and teachers sued on the grounds that the reading was in effect the introduction of a particular religion into the schools.
In a remarkable opinion, Judge Jack Jones ruled after a six-week trial that not only was ID a religious doctrine, and therefore not appropriate to the public schools, but that it was not good science, citing refutations of the arguments about complexity that had been made in his courtroom. This is the latest in a long series of defeats that creationists have suffered in the court system.
We maintain that it is reasonable, perhaps even desirable, to discuss different ways of knowing about the origin and evolution of life in classes on the history of ideas or concerning comparative religions or even on current events. However, we view efforts to eliminate the teaching of evolution or to promote the thinly veiled creationist agenda of ID proponents in a science classroom as misguided and a significant threat to the integrity of public science education. We argue that evolution is an essential unifying concept in biology and thus is a critical aspect of any scientific education. All students should be expected to understand the principle of evolution and to be familiar with the extensive observational evidence that scientists have discovered to support it, even if they don’t believe that evolution actually happened.
FRONTIERS
Molecular Evolution and Molecular Clocks
If evolution is really driven by the accumulation of mutations in DNA molecules, then one measure of how long it has been since two species shared a common ancestor is the difference in their genetic codes. The study of life’s history is now being aided by the molecular biologists, sometimes with surprising results.
Studies of human DNA reveal similarities that seem to indicate that all living humans have a single common ancestor—a woman who lived in Africa about 200,000 years ago and who has been named, appropriately enough, Eve. Similar studies have been used to sort out the human famil
y tree—to show, for example, that we are more closely related to chimpanzees than to the other great apes.
An aspect of molecular studies that has caused a great deal of controversy is the so-called molecular clock. Scientists who propose this idea believe that studies of DNA can not only clarify family relationships but can tell when branchings occurred in the evolutionary tree. Their idea is that changes in DNA occur regularly, like the ticking of a clock, so that if you know how many changes have taken place since two organisms shared a common ancestor, you can tell how long it’s been as well.
We expect that there will be a great many molecular contributions to evolutionary theory in the future, and, inevitable turf battles aside, that they will lead to new and exciting insights into our past.
Human Evolution
Although the evolution of humans is not, strictly speaking, more important than the evolution of any other organism, it has an intrinsic interest for members of our species. There are two hot fields of research in this area. One, centered in Africa, concerns finding the oldest human remains and tracing our own family tree to that of the distant ancestors of the apes. This is an intensely competitive field, since fossil finds are few and far between, and discoveries receive a lot of media attention.
Another busy field these days is the study of Neanderthal man, our closest cousin on the family tree. Neanderthal be came extinct only 35,000 years ago, and most of the debate centers on how like us Neanderthal were—whether they had speech, for example. As with other research in human evolution, our database is very small so that new discoveries create shock waves every now and then. Everything we know about Neanderthal man is based on the fossilized remains of about a hundred individuals.
A third group of researchers studies the distant ancestors of humans and the great apes, and particularly the question of where our branch of the family tree splits off from the others. This field of study receives less attention in the press than reports of early man, but it is equally fascinating to professionals. As so often happens with the fossil record, there are time gaps that make it very difficult to trace the evolution from the last generally accepted common ancestor to the earliest hominids.
Evolution and DNA
In addition to fossils, evidence for evolution is increasingly being found by examining the DNA of modern organisms. One type of evidence can be gotten simply by looking at the differences in the DNA between two such organisms and noting that the more differences there are, the longer it has been since the two shared a common ancestor. In this way, DNA differences can be used to construct a kind of family tree of living things. The fact that the family tree built in this way is identical to the family tree derived from fossils is strong evidence for evolution.
The process of natural selection can be thought of as a kind of “proofreader” for DNA. If a gene is functioning well, mutations are likely to make the gene less effective in its job. In this case, natural selection will act to eliminate individuals with the mutation from the population, and the most efficient form of the gene will be preserved for long periods in many kinds of organisms. Scientists have identified hundreds of genes, for example, that are the same in bacteria and higher animals—organisms that last shared a common ancestor two billion years ago.
Modern research is also starting to target the evolution of specific genes. For example, in human beings the sense of smell is less important than it is in other mammals. Scientists have been able to identify specific genes that (1) no longer operate in humans, and (2) are in the process of being erased by random mutations. By examining human DNA, in other words, we can actually catch genes in the process of disappearing. These so-called “fossil” genes are reminders of our mammalian ancestors.
CHAPTER NINETEEN
Ecosystems
LAKE VICTORIA, AFRICA’S LARGEST body of freshwater, was once the home of hundreds of species of fish. Among the most important to humans was the tilapia, a delicacy vital to the local economy. Africans harvested and sun-dried tons of the fish, which provided the principal source of protein for millions of lakeshore people.
In the 1960s British sportsmen introduced a new species into the lake—the Nile perch, a voracious predator that grows to several hundred pounds. At first the tilapia population survived perch predation by escaping to deep water where the perch’s visual hunting techniques don’t work. But the perch ate other species of fish that limited algae growth. Unchecked, the algae grew out of control, died, sank to the bottom, and decayed, thus destroying oxygen in the tilapia’s deep-water sanctuary. With the bottom zone uninhabitable, the unprotected tilapia population is now all but gone. The perch have also eliminated snail-eating fish, so snails, which carry dangerous parasites, have become a major health hazard.
The shores of Lake Victoria are nearly equally divided among Kenya, Uganda, and Tanzania. Millions of Africans in hundreds of lakeshore towns and villages have been affected by the changing lake ecology. Lake Victoria’s native fishermen have switched from tilapia to Nile perch, but the larger fish cannot be sun-dried effectively. The fishermen must roast the perch over wood fires. Now the lake’s shoreline has been stripped of trees, resulting in soil erosion and more lake damage. The introduction of one new species has drastically altered an entire ecosystem—an unintended result of man’s simple desire for better sport fishing.
This story illustrates a profound truth about living things:
All life is connected.
Living things grow in systems that process the energy and cycle the nutrients needed to support a community of organisms—complex arrangements we call ecosystems. Scientists describe and study ecosystems by mapping the transfer of energy and raw materials (minerals, soil, water) among living things and between living things and their environment.
At the base of the food chain in every ecosystem are self-sustaining organisms—plants and other photosynthetic life. Plants convert energy from the light of the sun to energy-storing molecules, and these molecules serve as the energy source not only for the plants, but for every other living thing. The energy moves up through levels of organisms—those that feed on plants, those that feed on those that feed on plants, and so on—in a complex food web. Eventually, energy leaves the ecosystem and is radiated into space, but the atoms in the molecules remain to be cycled through again and again.
THE HOUSE OF LIFE
Most biologists study life by tackling a small, manageable system—one organ, one cell, or even one molecule. But living systems never occur in isolation. Life requires the complex interaction of many organisms with their surrounding environment. Organisms cooperate and compete, eat or are eaten. Life on Earth, along with its nonliving environment, functions as a unit, obeying all the physical and biological principles described in earlier chapters. You have to study the whole integrated system if you want to understand our planet, and this is where the science of ecology (a word derived from the Greek term for house) enters the picture. Ecologists study ecosystems, so they concern themselves with all the organisms in a given area and their physical environment.
An ecosystem encompasses no fixed size. Almost any chunk of our planet that includes minerals, air, water, plants, animals, and microorganisms that interact will qualify. An ecosystem could be a swamp, a square yard of meadow, a sand dune, a coral reef, or an aquarium. Natural ecosystems seldom have sharp boundaries: forests merge into fields, shallow water grades into deep water.
Within an ecosystem, each organism fits like a gear in a complex machine. Each organism depends on its fellows, but performs necessary functions for them as well. Termites in a forest depend on trees to produce deadwood, for example, and the trees depend on the termites to clear the ground for new seedlings. The special place occupied by an organism in an ecosystem is called its ecological niche.
All living things on our planet exist in a thin layer at the surface, a layer that extends a few miles below the solid surface and a few miles into the air. We call this region the biosphere, and it can be thoug
ht of as Earth’s largest ecosystem.
One rule seems to emerge from studies of ecosystems, a rule that follows from the complexity of the web that connects living and nonliving things. It can be stated simply:
You can’t change just one thing in an ecosystem.
More grandiloquently, it is:
The Law of Unintended Consequences.
No matter how it’s stated, the rule comes down to this: in a complex system it is not always possible to predict what the consequences of any change will be, at least with the present state of knowledge. This means that seemingly small changes in ecosystems can cause large effects, while huge changes might leave the system pretty much as it was.
Having made this point, we should also note that life on Earth has survived many wild swings in environment in the past. Nature itself is constantly changing the global environment, so that change in and of itself is not necessarily a bad thing, and it’s certainly not unnatural. Nevertheless, the fact remains that we cannot presently predict with certainty what the ultimate effect of any given change will be.
ENERGY AND THE FOOD WEB
The sun provides the primary source of energy for life on Earth. Plants, plankton, and other green life use this radiant energy to convert carbon dioxide and water to energy-rich chemicals—simple carbohydrates—by the process of photosynthesis. Plants harness that chemical energy to produce the more complex molecules—proteins, lipids, and sugars—from which leaves and stems and flowers are made. Plants and other photosynthetic organisms are self-sustaining life. They are the primary producers of energy-storing molecules used by all living things, and scientists refer to them as the first trophic level in the environment. It is in the first trophic level that the food chain begins, and it is this level that supplies energy to all living things.
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