Modern humans (Homo sapiens ) are grouped with the mammals (class Mammalia) in the subphylum Vertebrata. Within the mammals, humans are assigned by taxonomists to the primates (order Primates), along with lemurs, monkeys, and apes. Humans are grouped by most taxonomists together with the surviving species of Hominoidea, or great apes (the others include two species of gorilla, two species of chimpanzee, orangutans, and simangs). However, all members of the human family can stand upright with no difficulty and are naturally bipedal , whereas apes are naturally quadrapedal, only standing upright when necessary. Humans are also the only surviving members of the family Hominidae and genus Homo. Extinct members of the genus include Homo habilis and Homo erectus. The Homo sp. probably evolved from one of several species of australopithecines: Australopithecus africanus, A. robustus, A. boisei, and the recently discovered A. garhi. "Lucy" (Australopithecus afarensis ) lived around 3.5 million years ago (abbreviated as mya) and may have been the ancestor of the evolutionary branch that led eventually to humans. It is not yet clear where the recently discovered Kenyanthropus platyops (who also lived around 3.5 mya) fits into the evolutionary sequence.
About 2.5 mya, the first humans, genus Homo, first appeared in the fossil record. For the first half million years, early representatives of Homo lived in loose association with australopithecines, from whom they differed in two important ways. First, they were somewhat larger and had much larger brains than australopithecines. Second, they used tools. Their tool use is the origin of the names for one species of these early humans, Homo habilis, or "handy" man. H. habilis lived in the dry savannas and forest edges, probably surviving for most of the year on roots, bulbs, and tubers. Although roots, bulbs, and tubers are very nutritious, digging them up from the hard, dry, savanna soils is difficult with only bare hands. Roots and tubers are also deficient in protein. So H. habilis would have needed to supplement the carbohydrate-rich diet of roots and tubers with high-quality protein, the best source of which is meat. H. habilis was a hunter and meat eater.
When you cannot run very fast and do not have big, sharp teeth, hunting large animals requires some form of social organization. So H. habilis individuals probably lived in small bands of closely related members. It is likely that some division of labor also existed, with the females doing most of the digging (something you could do while holding an infant) while the males hunted large game.
Fossils of another early member of our genus, Homo erectus ("upright" man) first appeared in Africa around 2 mya and spread rapidly into Asia. The fossils of H. erectus found in Asia were originally known as Java man or Peking man. H. erectus was as large as humans but had a heavier build. H. erectus also made another significant technological advancement, fire. They also had more sophisticated tools that were probably used for cleaning and cutting meat, for scraping hides, and as weapons. H. Erectus survived in many parts of the world until around a quarter million years ago.
The same evolutionary patterns established by the transition from Australopithecus to Homo erectus were extended even further with the evolution of Homo sapiens. The earliest members of our species had larger brains and smaller teeth than did H. erectus. Several types of (probably competing) H. sapiens existed at the same time. All were skilled big-game hunters, suggesting a high degree of social organization and, probably, language. Another distinctly human trait also appeared with H. sapiens. They apparently had religious practices and some concept of an afterlife. This led to burial rituals and the inclusion of tools, clothing, weapons, and food in the burial, presumably to aid the deceased in the afterlife.
One type of early Homo sapiens, Neanderthal, was widespread in Europe and Asia between 75,000 and 30,000 years ago. Neanderthals were short, robustly built, and had brains that were somewhat larger than modern humans. They used a wide variety of tools and were skilled hunters. However, an even more modern human, Cro-Magnon, appeared around 100,000 years ago. Cro-Magnon peoples and Neanderthals lived at the same time, but Neanderthals abruptly vanished from their range. Some biologists think Cro-Magnons exterminated Neanderthals, whereas others propose that interbreeding may have obliterated the differences. Cro-Magnon humans had even more sophisticated tools, modern language capabilities, and made extraordinary cave paintings.
Larger brains led inevitably to the evolution of human languages. Language, and the sophisticated social organization it makes possible, offers enormous evolutionary advantages. Not only were humans able to organize themselves into sophisticated hunting parties, they were also able to transmit information about other resources. Language also offers one other enormously important advantage. With language it is possible to transmit information from one generation to the next. The ability to pass along knowledge, traditions, rituals, and other information led to the development of culture. Cultural change can occur much more rapidly than genetic change. A cultural trait can spread through a population in less than one generation. The problem of cultural transmission is how to pass along cultural norms efficiently without being rigid and stifling the creativity necessary for a population to survive adverse changes in the environment.
Unique cultural traditions have been identified in the tools, weapons, and other implements found associated with human fossils. Along with cultural traditions, the domestication of plants and animals also spread rapidly. As a result most human societies eventually became sedentary. Agriculture and pastoralism (herding of domestic animals) replaced hunting and gathering. Agriculture and pastoralism led to cities, expanded food supplies, stratified societies, and the rapid growth of the human population.
Agriculture was independently invented three times at different places in the world. Agriculture was first discovered in the Middle East about 11,000 years ago and spread from there throughout Europe. From Middle Eastern agriculture we get cereal grains, grapes, and olives. European agriculture gave us rye, cabbage, celery, and carrots. Domesticated animals included cattle, sheep, goats, horses, pigs, dogs, cats, and chickens. Agriculture also developed in east Asia about the same time. From Asian agriculture we get rice, soybeans, citrus fruits, and mangoes.
When humans first entered the New World, they did not bring agriculture or domesticated animals with them, except for dogs. So agriculture developed a third time in the New World and gave us corn, tomatoes, kidney and lima beans, peanuts, potatoes, chili peppers, and squash. Domesticated animals were rare in the New World and included only llamas, alpacas, and turkeys.
Human Population Growth
Humans have successfully moved into every available nook and cranny on Earth. Our sophisticated technology allows us to survive comfortably where no other mammal or any complex organism could survive. Humans spend the winter at the South Pole. Humans live on mountaintops and in arid deserts. These are mere curiosities, but it is obvious that humans are able successfully to make a living in a wider variety of habitats and under a broader range of environmental conditions than can any other animal on Earth.
For the first few million years of our evolution, humans lived in small groups and survived by hunting and gathering. The invention of agriculture allowed human populations to grow rapidly. They are still growing. If the number of humans on Earth is plotted against time, the curve stays essentially horizontal until about 1000 C.E. At that time, there were less than 100 million people in the entire world. From 1000 C.E. to 2000 C.E., the population growth curve turned sharply upward and now appears almost vertical. It took 2 million years to reach the first billion people, 130 years to reach the second billion, 30 years to reach the third billion, 15 years to reach the fourth billion, and only 12 years to reach five billion. The population of the world passed six billion in September 1999 and in March 2001 was over 6,137,748,000. By 2010 the world's population will pass seven billion.
This rapid population growth and the spread of humans to every part of the globe have profoundly altered the environment. Obviously the population of Earth cannot grow indefinitely. At some point, resources will run out and population growth will be limited. Biologists wonder what the carrying capacity of Earth is and what the quality of life would be like if the human population were to be allowed to increase to that point.
Carrying capacity is the maximum population of a given species that an ecosystem can support for an extended period of time. Every habitat, ecosystem, or biome has a carrying capacity of any particular species. Humans have moved into every portion of Earth and inhabit a variety of different ecosystems. Discussions of carrying capacity for human population must include the whole Earth as an ecosystem. There is much debate and discussion of Earth's carrying capacity. Many feel that Earth is already overpopulated and that drastic measures must be taken immediately to reduce the population and to reduce resource consumption.
Humans have already transformed or degraded 40 to 50 percent of Earth's land surface. Humans use 8 percent of the total productivity of oceans. The percentage is much higher in the areas of concentrated productivity, such as continental shelves. Humans have already increased atmospheric carbon dioxide by 30 percent. On many islands, over one-half of the species have been introduced by humans, often devastating native populations. Over 20 percent of bird species have become extinct since 1800 as a result of human activity. Over 22 percent of marine fisheries are overexploited and are now in decline. Another 44 percent are at the limit of exploitation.
The biggest problem humankind may have to face in the near future is the availability of clean, fresh water. Humans already use over one-half of the available fresh water. Some experts predict that in the twenty-first century competition for water resources will come to dominate local, national, and international politics. These experts predict that the competition for water resources will be much more severe and dangerous than the present competition for energy resources.
Land resources are also limited and cannot support unlimited population growth. Cropland, rangeland, pasture, and forests are all under pressure. Most land suitable for farming is already being farmed. Increases in agricultural productivity through higher yield crops and more efficient farming practices have allowed agricultural production to increase more rapidly than the population. Most experts think that this cannot continue indefinitely.
Human Impact on the Environment
Humans interact with both the living and nonliving factors in our environment. Environmental degradation occurs when a potentially renewable resource such as soil, grassland, forest, or wildlife is used at a rate faster than the resource can be replaced. The resource becomes depleted and environmental degradation occurs. If the rate of use of the resource remains high, the resource can become nonrenewable on a human timescale or it can even become nonexistent (extinct). Worldwide, species are disappearing at a rate greater than the rate of species loss during any of the mass extinctions Earth has undergone.
Not only are species being lost at an alarming rate, biodiversity is also being lost at the ecosystem level because of environmental degradation. Tropical forests are recognized as the most diverse ecosystems on Earth and are experiencing the highest rate of ecosystem loss, but temperate habitats are also suffering degradation. Because the temperate parts of the world were settled first, it is in these areas that the loss of biodiversity has been greatest.
Who is responsible for degrading the environment? We all are. Ordinary human activity from even the most responsible individuals inevitably pollutes and degrades the environment to some extent. We directly degrade the environment when we consume resources (burning wood in a fireplace, for example) and indirectly when resources are extracted and transformed into products we need or want.
see also Biodiversity; Population Dynamics; Populations.
Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishing,1989.
Ehrlich, Paul R. The Population Bomb. New York: Ballantine Books, 1971.
Findley, Rowe "Will We Save Our Own?" National Geographic 178, no. 3(1990):106-136.
Kohm, Kathryn A., ed. Balancing on the Brink of Extinction: The Endangered Species Act and Lessons for the Future. Washington, DC: Island Press, 1991.
Miller, G. Tyler, Jr. Living in the Environment, 6th ed. Belmont, CA: Wadsworth,1990.
Odum, Eugene Pleasants. Fundamentals of Ecology, 3rd ed. Philadelphia, Saunders,1971.
Purves, William K., and Gordon H. Orians. Life: The Science of Biology. Sunderland, MA: Sinauer Associates, 1987.
Terborgh, John. Where Have All the Birds Gone? Princeton, NJ: Princeton University Press, 1989.
Sustainable development is a form of social change that includes recognizing that maintenance of natural resources is a basic human need. The idea of resource sustainability emerged in the late nineteenth century with regard to renewable resources such as forests and fisheries. At the start of the twenty-first century, the concept encompasses many other ideas, including population control as one of the keys to maintaining Earth's biosphere.
Human population refers to the number of people living in a particular area, from a village to the world as a whole. A secondary meaning of population is the inhabitants themselves, but in most uses population means numbers.
No one knows the population of the earliest humans, but there may have been only a few tens of thousands of individuals when the species Homo sapiens first emerged 200,000 years ago. Today more than 6 billion human beings inhabit the earth. Three-fifths of them live in one continent, Asia, with the rest occupying every continent except Antarctica.
The overwhelming bulk of human population growth has occurred since the Industrial Revolution began, more than half since 1950. All but a small percentage of the roughly 80 million people added to world population each year live in the world's developing countries, which are home to 80 percent of humanity and more than 95 percent of world population growth. In Europe and Japan, small average family size and relatively modest immigration levels are leading to a leveling of, and even decreases in, population. In the United States, Canada, and Australia, slightly larger families and higher levels of immigration make for continued population growth.
World population grows because births significantly outpace deaths on average. This imbalance occurs not because women are having more children than they once did—quite the reverse—but because improved sanitation and health mean that many more children than in the past survive to become parents themselves. Human reproduction is such a success story that some analysts believe that today's large and ever-increasing population growth threatens the earth's support systems and contributes to global poverty.
Debate on this question has raged since at least the 1800s. Some economists and other social scientists argue that higher populations provide more human resources for solving problems and producing wealth. Most physical and biological scientists, by contrast, argue that key natural resources—fresh water, cropland, forests, and fisheries, for example—are increasingly strained by burgeoning human demands. Rising natural resource consumption by individuals also boosts these demands. The long-term growth of human population clearly has been an especially significant factor in human-induced climate change, species extinction, the loss of forests, and other environmental problems. But scientists and other analysts have been unable to agree on population's exact role in environmental change. Many other factors, from consumption patterns to government policies to the unequal distribution of power and wealth, also influence the environment.
One clear trend in human population is that its growth is slowing down. Women and men increasingly want to have later pregnancies and smaller families than did their own parents. Governments increasingly provide the health services that allow couples to plan their families. For some countries, this trend raises questions about how societies will cope with lower proportions of young and working people. For the world as a whole, however, births are likely to outnumber deaths for decades to come, and human population will continue to grow.
see also Biodiversity; Desertification; Extinction; Global Climate Change
Cohen, Joel E. How Many People Can the Earth Support? New York: Norton, 1995.
Mazur, Laurie, ed. Beyond the Numbers: A Reader on Population, Consumption, and the Environment. Washington, DC: Island Press, 1995.
Robey, Bryant, et al. "Fertility Decline in Developing Countries." Scientific American 269, no. 6 (December 1993): 60–67.