Evolution of humans

The biochemical and behavioral evidence for an evolutionary relationship between humans and apes is amply confirmed by the fossil record of human ancestors, mainly from Africa. The evolutionary tree of humanity can be traced back some 33 million years when ancestral apes and monkeys had evolved. The earliest erect-walking humans evolved some five to ten million years ago from the stock that also gave rise to gorillas and chimpanzees. The earliest morphologically modern humans, from which all living populations, date back only about 200,000 years.

Early Apes and Monkeys

The fossil record of human evolution in Africa can be traced back to the Oligocene and the Eocene epochs, some 30 to 50 million years ago, in the al-Fayyūm depression of Egypt. Some of the creatures fossilized here may have been ancestral to modern monkeys. Others, such as Aegyptopithecus, had the low-crowned, five-cusped molars of modern apes and humans. Aegyptopithecus, a small creature with an average weight of 6 to 7 kilograms, was monkeylike in body and may have moved on all fours through trees in search of fruit and leaves. The fossils of similar, weakly differentiated monkey-apes have been found in rocks of the Eocene age in Burma (Pondaungia and Amphipithecus) and of the Oligocene age in Bolivia (Branisella). The South American forms evolved in isolation into New World monkeys (howlers, marmosets, and capuchins). Evolution took a different course in Africa and Asia, culminating in the Old World monkeys (vervets, colobuses, and baboons), as well as apes and humans.

By the Early Miocene, some 23 to 17 million years ago, there were a variety of early apes and monkeys in East Africa. They are particularly well known as fossils from Rusinga Island and nearby localities in southwestern Kenya. The fossils of Old World monkeys (Victoriapithecus) have been found, as have the fossils of a much larger group of monkeylike apes referred to as dryopithecines (Proconsul, Rangwapithecus, Dendropithecus, Limnopithecus). The monkeylike group had four ridged cusps on their molars, which may reflect a preference for eating leaves. The dryopithecines were apelike in having five low cusps, which suggests that their diet included more fruit. The dryopithecines were nevertheless monkeylike in the way they walked on all fours and climbed trees. Their habitat was dry tropical forest, like that covering large areas around the margin of the jungles of the Congo Basin. These forests were less lush than rain forests, and they offered more local and seasonal variation in food and shelter.

By the Middle Miocene, some 14 million years ago, there appeared a new group of apes. These ramapithecines are distinguished primarily by canines in females reduced almost to human proportions. The males, however, were strongly built and had well-developed canines like other apes. In other respects, they can be pictured as monkey-like ancestors to orangutans. An early African ramapithecine (Kenyapithecus), known from fossils in Kenya, is found along with early monkeys and dryopithecine apes in almost equal abundance. By this time, dryopithecines had waned in abundance and diversity as monkeys became more prominent. Great environmental changes occurring then would have favored leaf-eating monkeys over fruit-eating dyropithecines. Fossil soils and grasses in Kenya indicate the appearance of wooded grassland. This open vegetation, often called savanna, developed in response to a drier climate than had existed earlier during the Miocene. By this time, both dryopithecines and ramapithecines had dispersed into Europe and Asia. The European apes became extinct 10 million years ago, but ramapithecines evolved into orangutans and dryopithecines into gibbons in Southeast Asia.

Development of the Erect Stance

In Africa, meanwhile, monkeys continued to evolve and diversify while ramapithecines and dryopithecines declined in importance. A small fragment of a possible ancestor of the gorilla was found in rocks about eight million years old in the Samburu Hills of Kenya. Gorillas and chimpanzees share a distinctive series of anatomical features in their hands useful for knuckle-walking. These great apes probably had evolved from an African ramapithecine. By five million years ago, this same evolutionary lineage had given rise to the first australopithecines. These erect-walking creatures were short (1 to 1.5 meters), and, by human standards, their brains were small (cranial capacity of only 400 to 500 cubic centimeters). They can be pictured as erect-walking chimpanzees.

Scientists continually debate the oldest species considered a hominin (the revised term for modern humans and close relatives, sometimes still referred to as "hominid"). Ardepithecus ramidus from Ethiopia is estimated to be 4.4 million years old, while discoveries of other bipedal species in the early twenty-first century proposed origins of six or seven million years ago. These were small facultative bipeds that lacked some chimpanzee specializations and so were clearly on the hominin line. Early australopithecines are well understood from a spectacular series of skeletal remains (Australopithecus afarensis) from Harar, Ethiopia, and a set of trackways from Laetoli, Tanzania, both about three to four million years old. Both localities also have yielded fossilized soils, pollen, and mammals of a mosaic of woodland and wooded grassland similar to that still seen in many East African game parks.

Erect stance appears to be the most important evolutionary innovation separating humans from apes, and its origin continues to attract speculation. The earliest evidence of this is a thigh bone from the Awash Valley of Ethiopia, dated between 4.0 and 3.5 million years ago. According to a series of martial hypotheses, erect stance was an adaptation for hunting big game in open savanna. Not only did it leave hands free to throw spears, but it also enabled early humans to see potential prey and predators over the tall grass. A second set of hypotheses emphasizes the innovative use of new resources in open habitats. For example, a dietary shift toward small seeds would require precise manipulation, and the need to locate carcasses and to scavenge would require greater mobility. Erect stance would help to meet both these needs. A 2007 study also suggested that bipedalism is more energy efficient. A third set of hypotheses emphasizes the nurturing side of human nature, such as the need to carry food and other materials back to a home base. Nurturing hypotheses are preferred by many paleoanthropologists over the other hypotheses because, for one thing, the earliest erect-walking australopithecines had curved fingers and toes and so retained a facility for climbing trees. In addition, savanna habitats were a part of the African environment and were exploited by a great variety of monkeys for at least 10 million years before the advent of erect stance.

Development of Tool Use

From 2.5 to 1 million years ago, at least two distinct kinds of australopithecines coexisted in savanna mosaic vegetation then covering many parts of Africa. Some were strongly built and weighed 60 kilograms on average. These robust forms (Australopithecus robustus, A. boisei, or Paranthropus) had large grinding molar teeth and heavily muscularized skulls. They may have eaten large quantities of tough vegetable material, such as seeds and tubers of the open savanna, or low-grade browse of woodland and forest. Others were smaller and lightly built, weighing an average of 30 kilograms. These slight, slender (gracile) forms (Australopithecus africanus, Homo habilis) had smaller teeth, indicating a softer diet that included meat, fruits, and vegetables. Crudely fashioned pebble tools of the Oldowan culture are found as old as 2.6 million years. Although their makers are not known with certainty, they are widely attributed to the lightly built species Homo habilis, in large part because of that group's cranial capacity (about 650 to 680 cubic centimeters) compared with that of the robust forms (about 500 to 530 cubic centimeters) that lived at the same time. This difference in brain size—and presumed intelligence—is all the more striking because of the greater body size of the robust forms.

About 1.5 million years ago, a new suite of stone tools of the Acheulian tradition appeared, along with a new and more human species, Homo erectus. At about this time, earlier australopithecine species declined in abundance and became extinct. The new humans were taller (1.8 meters) and heavier (55 kilograms), with markedly enlarged brains (cranial capacity of 750 to 1,225 cubic centimeters). Their molar teeth were even more reduced in size than in their gracile australopithecine ancestors. Their tools and butchery sites indicate organized big-game hunting. Fruit and vegetables still may have played a large role in the diet, as in modern hunter-gatherer societies. Homo erectus may have used fire as long as 1.4 million years ago in Africa and certainly used fire by 500,000 years ago. These humans spread out of Africa into Spain, Europe, China, and Indonesia. Archaic humans of the species Homo sapiens evolved from this stock by about 500,000 years ago in North Africa or the Middle East.

The Neanderthals (Homo neanderthalensis; believed by some scientists to be a subspecies of Homo sapiens) evolved during the Ice Age in northern Europe. DNA evidence as of 2014 clarified that Neanderthals interbred with Homo sapiens, as the two species coexisted for a considerable time before modern humans began to dominate for a variety of proposed reasons. This interbreeding was shown in 2016 to have extended as far back as 100,000 years ago, with DNA flowing not only from Neanderthals to modern humans but also in the opposite direction. This genetic evidence suggests that humans migrated out of Africa around that same time period, much earlier than previously thought, although all modern humans are descended from later waves of migration.

In addition to Neanderthals, several discoveries in the twenty-first century led to proposals of other Homo species or subspecies that, if accurate, further complicate the human evolutionary family tree. These include Homo floresiensis, discovered in Indonesia in 2003, Homo gautengensis, found in South Africa in 2010, the Denisova hominin, discovered in Russia in 2010, and the Red Deer Cave people, discovered in China in 2012. Research in this field is ongoing and often controversial.

Appearance of Homo sapiens

Remains of the earliest morphologically modern humans (Homo sapiens) are found in African deposits ranging back in age to about 120,000 years. These skulls have the prominent chin and highly inflated brain case of modern humans (cranial capacity of 1,276 to 1,400 cubic centimeters). The remains indicate that these humans used new and more finely tooled stone implements; they also appear to have been hunters and gatherers. They diffused into Europe and Asia, extending to Australia by 40,000 years ago and into North and South America perhaps as long ago as 30,000 years.

All modern humans are thought to have descended from this African stock, which replaced the earlier forms (Homo erectus and archaic Homo sapiens) throughout their range. It was this new wave of African emigrants of 30,000 years ago that brought to the world the first art in the form of cave paintings and portable statuettes. They may also have practiced a crude form of animal husbandry by controlling horse herds in a manner comparable to the shepherding of reindeer by the Lapps of Finland. Since that time, there has been some limited human evolution, particularly a reduction of tooth size in response to softer cooked and processed foods. Physical evolution has been surpassed in the modern era by dramatic and increasingly multifaceted cultural evolution. However, the emergence of resistance to newly introduced diseases, such as those that transfer from animal populations to humans, reveals continuing drivers of human evolution.

Study of Human Evolution

The most important evidence of human evolution is fossil remains. Many thousands of such fossils have been found over the years, but scientific excitement still is aroused by the discovery of an especially complete skull or skeleton. The bones are compared with one another and with bones of living apes and humans in order to reconstruct the modifications to bone shape that have evolved over geological time and to determine what they may indicate concerning the lifestyle of early humans. The use of X-rays can reveal bone structures and evidence of stress or disease not visible on the surface of the bones. Medical computerized axial tomography (CAT) scans also are used to explore the interior spaces of fossil skulls. Study under the scanning electron microscope can distinguish scrapes on bone surfaces made by cutting with tools from those made by gnawing. Images of microscopic grooves and pits on the surface of teeth can be a clue to the hardness and grit content of the diet of human ancestors.

Also important to the study of human evolution is an evaluation of the geological circumstances of the fossils: their distribution in the rocks; the origin of the entombing ancient soil, stream or lake deposits, or volcanic ash; and the affinities of associated fossilized pollen, leaves, snails, or mammals. These separate lines of evidence are useful for understanding the lifestyles of early humans and for reconstructing the stages of death, burial, and fossilization of human remains.

Consistent methods of determining geological age are essential. The traditional methods of dating deposits by their fossil content (biostratigraphy) or by stone tools have been widely used but lack fine resolution. Correlation of lavas and ashes by distinct differences in chemical composition and their dating by isotopic methods, such as potassium-argon and rubidium-strontium dating, have become especially important for calibrating human evolution.

The methods of modern molecular biology show the evolutionary relationships and distances between humans, apes, and monkeys. Large complex molecules such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) have evolved over time as small parts of them changed. The degree of similarity of such molecules in different organisms has been shown to be proportional to the geological time that has elapsed since they had a common ancestor. Monkeys and humans are quite different in overall DNA compared to the genetic relationship between humans and chimpanzees. These evolutionary distances can be used to estimate geological time since evolutionary divergence. The fossil record shows that a common ancestor of humans and monkeys existed no more than 33 million years ago. Using this datum to calibrate the DNA data, the common ancestor of chimpanzees and humans, not yet known from the fossil record, probably lived at least 7 million years ago. Similar kinds of evidence, including larger-scale genomic studies, corroborate the theory that modern humans originated in Africa about 200,000 years ago.

The study of human evolution continued into the first decades of the twenty-first century. In the 2020s, several important studies revealed new information about evolution covering topics from food consumption, the domestication of animals, family and relationships, genetic inheritance, the spread of disease, and human population bottlenecks. In 2023, a study indicated that humans could have evolved from different times and locations in Africa, rather than from one time and place as previously thought.

Principal Terms

acheulian: a cultural tradition distinguished by stone tools characterized by large (about 15-centimeter), crudely fashioned, leaf-shaped hand axes

apes: mammals somewhat like monkeys but lacking a tail and with five low cusps on their molars instead of four sharp ones; they include gibbons, orangutans, chimpanzees, and gorillas

australopithecines: erect-walking early human ancestors with a cranial capacity and body size within the range of modern apes rather than of humans

cranial capacity: the internal volume (in cubic centimeters) of the brain case of a skull

dryopithecines: extinct kinds of apes that walked and climbed like monkeys but that lacked the four-cusped molars of monkeys, the tree-swinging specializations of gibbons and orangutans, and the knuckle-walking specializations of chimpanzees and gorillas

knuckle-walking: a form of locomotion found in gorillas and chimpanzees whereby they walk on all fours, with feet facing forward but hands curled backward so that the knuckles are placed on the ground

Oldowan: a cultural tradition distinguished by stone tools characterized by rounded pebbles crudely chipped to a sharp edge on one side

ramapithecines: extinct kinds of apes in which the males had prominent canines as in other apes, but the females had smaller canines as in humans; otherwise, these creatures were like dryopithecines or orangutans

Bibliography

Argue, D., et al. “Homo floresiensis: A Cladistic Analysis.” Journal of Human Evolution, vol. 57, 2009, pp. 623–639.

Benton, Michael J. The Rise of the Mammals. Quantum, 2005.

—. Vertebrate Paleontology. 4th ed., Wiley, 2015.

Day, Michael H. Guide to Fossil Man. 4th ed., U of Chicago P, 1993.

Feltman, Rachel. “Humans and Neanderthals May Have Interbred 50,000 Years Earlier than Previously Thought.” The Washington Post, 17 Feb. 2016, www.washingtonpost.com/news/speaking-of-science/wp/2016/02/17/humans-and-neanderthals-may-have-interbred-50000-years-earlier-than-previously-thought/. Accessed 20 Nov. 2024.

Fu, Qiaomei, et al. “Genome Sequence of a 45,000-Year-Old Modern Human from Western Siberia.” Nature, vol. 514, 2014, pp. 445–449.

Johanson, Donald C., and Maitland A. Edey. Lucy: The Beginnings of Humankind. Touchstone, 1990.

King, Geoffrey, and Geoff Bailey. “Tectonics and Human Evolution.” Antiquity, vol. 80, 2006, pp. 265–286.

Leakey, Richard E. The Making of Mankind. Dutton, 1981.

McKee, Jeffrey K., Frank E. Poirier, and W. Scott McGraw. Understanding Human Evolution. 5th ed., Prentice Hall, 2004.

McRae, Ryan, and Briana Pobiner. “Fourteen Discoveries Made About Human Evolution in 2022.” Smithsonian Magazine, 27 Dec. 2022, www.smithsonianmag.com/smithsonian-institution/fourteen-discoveries-made-about-human-evolution-in-2022-180981344/. Accessed 20 Nov. 2024.

Sockol, Michael D., David A. Raichlen, and Herman Pontzer. “Chimpanzee Locomotor Energetics and the Origins of Human Bipedalism.” PNAS, vol. 104, no. 30, 2007, pp. 12265–12269.

Tobias, Philip V. “Major Events in the History of Mankind.” Major Events in the History of Life, edited by J. William Schopf, Jones, 1992.

Wicander, Reed, and James S. Monroe. Historical Geology. 8th ed., Cengage Learning, 2015.

Zimmer, Carl. "Study Offers New Twist in How the First Humans Evolved." The New York Times, 24 May 2023, www.nytimes.com/2023/05/17/science/human-origins-africa.html. Accessed 20 Nov. 2024.