Human evolution: the story on HearLore

Human evolution

The earliest evidence of human ancestry does not come from a fossil of a giant ape, but from a tiny skull found in a limestone quarry in 1856 that was initially dismissed as a diseased modern human. This specimen, known as the Neanderthal, sparked the first great debates about human origins, yet it was only decades later that the true story began to unfold. The real journey starts much earlier, over 65 million years ago, when the first primate-like mammals emerged from the ashes of the dinosaur extinction. One of the oldest known species, Plesiadapis, roamed North America, while Archicebus lived in China, hinting at a widespread distribution of early primates across the globe. These creatures were small, nocturnal, and lived in the tropical forests of the Paleocene and Eocene epochs, far removed from the savannas that would eventually shape our species. The fossil record from this period is sparse, leaving many gaps in our understanding of how these early primates evolved into the diverse array of species that would eventually include humans. However, the surviving tropical populations of primates, particularly those found in the Faiyum depression of Egypt, gave rise to all extant primate species, including the lemurs of Madagascar and the great apes of Africa and Asia. The divergence of the human lineage from other great apes began approximately 8 to 9 million years ago, when hominins parted ways from the Gorillini tribe, and later separated from the Pan genus, which contains chimpanzees and bonobos, between 4 and 7 million years ago. This split was not a single event but a complex process of divergence that occurred over millions of years, with the earliest hominins adapting to drier environments outside the equatorial belt. The fossil record of this period is limited, but species like Sahelanthropus tchadensis and Orrorin tugenensis provide crucial evidence of the transition from arboreal life to bipedalism. These early hominins, such as Ardipithecus, developed a suite of anatomical and behavioral adaptations that set them apart from their ape ancestors, including reduced canine teeth and a more human-like brain structure. The evolution of bipedalism was a pivotal moment in human history, freeing the hands for tool use and allowing early hominins to travel long distances across the savanna. This adaptation was not without its costs, as the smaller birth canal of bipedal apes made human birth more difficult and led to the relative immaturity of human offspring. The development of bipedalism also had significant effects on the female reproductive cycle, leading to the evolution of menopause and the grandmother hypothesis, which suggests that elderly women could better pass on their genes by taking care of their daughter's offspring. The transition from tree-dwelling to ground-dwelling life was a gradual process that involved changes in the skeletal structure, including the shape of the pelvis, the alignment of the big toe, and the position of the foramen magnum. These changes allowed early hominins to walk upright, run long distances, and eventually spread out of Africa to populate the rest of the world. The story of human evolution is one of adaptation, survival, and the constant interplay between biological and cultural factors that have shaped our species over millions of years.

When did the first primate-like mammals emerge from the dinosaur extinction?

The first primate-like mammals emerged from the dinosaur extinction over 65 million years ago. These creatures were small, nocturnal, and lived in the tropical forests of the Paleocene and Eocene epochs.

When did the human lineage diverge from other great apes?

The divergence of the human lineage from other great apes began approximately 8 to 9 million years ago. This split occurred when hominins parted ways from the Gorillini tribe and later separated from the Pan genus between 4 and 7 million years ago.

When were the oldest known tools created and by which species?

The oldest known tools date back 3.3 million years and were likely created by Australopithecus in West Turkana, Kenya. These simple flakes of stone marked the beginning of a technological revolution that eventually led to the development of complex tools.

When did Homo erectus leave Africa and spread to other continents?

Homo erectus was the first hominin to leave Africa, spreading throughout Africa, Asia, and Europe between 1.8 million years ago and 108,000 years ago. This migration was closely related to fluctuating periods of climate change and the closing of savannah corridors.

When did modern humans evolve in Africa and migrate out of the continent?

Modern humans evolved in Africa approximately 300,000 years ago and migrated out of the continent some 50,000 to 100,000 years ago. This migration led to the replacement of local populations of Homo erectus, Denisova hominins, and Neanderthals.

When was the genetic revolution in human evolution studies initiated?

The genetic revolution in studies of human evolution started when Vincent Sarich and Allan Wilson published their seminal paper in 1967. Their work used immunological cross-reactions to estimate the divergence time of humans and apes as four to five million years ago.

The Hand That Made Us

The oldest known tools, dating back 3.3 million years, were not made by a human ancestor but by a species that lived in West Turkana, Kenya, and were likely created by Australopithecus. These simple flakes of stone marked the beginning of a technological revolution that would eventually lead to the development of complex tools and the expansion of the human brain. The next oldest stone tools, found in Gona, Ethiopia, date to about 2.6 million years ago and are considered the beginning of the Oldowan technology. These tools were made by early Homo species, such as Homo habilis, which lived from about 2.8 to 1.4 million years ago. The name Homo habilis, meaning 'handy man', was bestowed by discoverer Louis Leakey, who recognized the significance of these early tools in the evolution of the human species. The development of tool use was a crucial adaptation that allowed early hominins to process more energy-rich plant products and hunt for meat, which in turn supported the growth of larger brains. The brain of a modern human consumes about 13 watts of power, a fifth of the body's resting power consumption, and the evolution of tool use was closely linked to the development of the human hand. The ulnar opposition, the ability to touch the little finger with the thumb, is unique to the genus Homo and facilitates the precision grip and power grip that underlie all skilled manipulations. This anatomical feature, which is not seen in human fossils older than 1.8 million years, allowed early humans to make and use complex tools with greater dexterity and strength. The evolution of the hand was accompanied by changes in the shoulder blades, which allowed human ancestors to throw objects with greater force, speed, and accuracy. These changes in the hand and shoulder were crucial for the development of hunting techniques and the ability to defend against predators. The use of tools also stimulated the expansion of the human brain, as the need to create and use tools required increased cognitive abilities. The brain of early Homo species, such as Homo habilis, was slightly larger than that of chimpanzees, and the pattern of encephalization continued in Homo erectus, which had a brain size of about 850 cubic centimeters. This increase in brain size was accompanied by changes in the structure of the brain, including the development of the temporal lobes, which contain centers for language processing, and the prefrontal cortex, which has been related to complex decision-making and moderating social behavior. The evolution of the brain was also linked to the development of cooking, which released increased nutritional value and allowed for the growth of larger brains. The control of fire and the use of cooking were key adaptations that separated Homo from tree-sleeping Australopithecines, and they were crucial for the survival and expansion of early human populations. The development of tool use and the expansion of the human brain were closely linked to the evolution of social behavior, as the need to cooperate and communicate with others drove the development of language and complex social structures. The story of the hand that made us is one of adaptation, innovation, and the constant interplay between biological and cultural factors that have shaped our species over millions of years.

The Great Migration

The first fossils of Homo erectus were discovered by Dutch physician Eugene Dubois in 1891 on the Indonesian island of Java, and he originally named the material Anthropopithecus erectus, considering it a chimpanzee-like fossil primate. Years later, the German physician and paleoanthropologist Franz Weidenreich compared the characters of Dubois' Java Man with the characters of the Peking Man, and concluded that all these specimens should be gathered in a single species of the genus Homo, the species Homo erectus. Homo erectus lived from about 1.8 million years ago to about 108,000 years ago, and this population appears to have died out when the savannah corridors closed and tropical jungle took over. The early phase of Homo erectus, from 1.8 to 1.25 million years ago, is considered by some to be a separate species, Homo ergaster, or as Homo erectus ergaster, a subspecies of Homo erectus. Many paleoanthropologists now use the term Homo ergaster for the non-Asian forms of this group, and reserve Homo erectus only for those fossils that are found in Asia and meet certain skeletal and dental requirements. In Africa, in the Early Pleistocene, 1.5 to 1 million years ago, some populations of Homo habilis are thought to have evolved larger brains and to have made more elaborate stone tools, and these differences and others are sufficient for anthropologists to classify them as a new species, Homo erectus. This species also may have used fire to cook meat, and Richard Wrangham notes that Homo seems to have been ground dwelling, with reduced intestinal length, smaller dentition, and brains swollen to their current, horrendously fuel-inefficient size. The control of fire and cooking, which released increased nutritional value, was the key adaptation that separated Homo from tree-sleeping Australopithecines. Homo erectus was the first of the hominin line to leave Africa, spreading throughout Africa, Asia, and Europe between 1.8 million years ago and 108,000 years ago. The migration of Homo erectus out of Africa was closely related to fluctuating periods of climate change, and recent evidence suggests that humans may have left Africa half a million years earlier than previously thought. A joint Franco-Indian team has found human artifacts in the Siwalk Hills north of New Delhi dating back at least 2.6 million years, which is earlier than the previous earliest finding of genus Homo at Dmanisi, in Georgia, dating to 1.85 million years. Although controversial, tools found at a Chinese cave strengthen the case that humans used tools as far back as 2.48 million years ago. This suggests that the Asian Chopper tool tradition, found in Java and northern China, may have left Africa before the appearance of the Acheulian hand axe. The dispersal of modern Homo sapiens was a complex process that involved multiple migrations out of Africa, and the genetic evidence suggests that these dispersals are closely related to fluctuating periods of climate change. The multiregional hypothesis, which proposed that the genus Homo contained only a single interconnected population as it does today, was challenged by the out of Africa model, which proposed that modern Homo sapiens speciated in Africa recently and the subsequent migration through Eurasia resulted in the nearly complete replacement of other Homo species. The out of Africa model has been developed by Chris Stringer and Peter Andrews, and it is supported by genetic evidence that suggests that modern humans evolved in Africa approximately 300,000 years ago and migrated out of the continent some 50,000 to 100,000 years ago. The migration of modern humans out of Africa was a pivotal moment in human history, and it led to the replacement of local populations of Homo erectus, Denisova hominins, Homo floresiensis, Homo luzonensis, and Homo neanderthalensis, whose ancestors had left Africa in earlier migrations. The story of the great migration is one of adaptation, survival, and the constant interplay between biological and cultural factors that have shaped our species over millions of years.

The Lost Cousins

In 2008, archaeologists working at the site of Denisova Cave in the Altai Mountains of Siberia uncovered a small bone fragment from the fifth finger of a juvenile member of another human species, the Denisovans. This discovery raised the possibility that Neanderthals, Denisovans, and modern humans may have co-existed and interbred, creating a much more complex picture of humankind during the Late Pleistocene than previously thought. The genetic sequencing of a 40,000-year-old human skeleton from Romania showed that 11% of its genome was Neanderthal, implying the individual had a Neanderthal ancestor 4 to 6 generations previously, in addition to a contribution from earlier interbreeding in the Middle East. All modern non-African humans have about 1% to 4% of their DNA derived from Neanderthals, and evidence has also been found that as much as 6% of the DNA of some modern Melanesians derive from Denisovans, indicating limited interbreeding in Southeast Asia. The existence of this distant branch creates a much more complex picture of humankind during the Late Pleistocene than previously thought, and it suggests that the evolution of humans was not linear but weblike. Neanderthals and Denisovans were not inferior to modern humans, and they had their own adaptations to the environments in which they lived. Neanderthals had significantly larger brains, as shown from brain endocasts, casting doubt on their intellectual inferiority to modern humans, and they had superior adaptation to cold environments, with a surface to volume ratio even lower than that among modern Inuit populations. Neanderthals also had larger brains, as shown from brain endocasts, casting doubt on their intellectual inferiority to modern humans, and they had better visual acuity than modern humans, useful in the dimmer light of glacial Europe. The larger size of the Neanderthal orbital chamber and occipital lobe suggests that they had a better visual acuity than modern humans, useful in the dimmer light of glacial Europe. Neanderthals may have had less brain capacity available for social functions, and inferring social group size from endocranial volume suggests that Neanderthal groups may have been limited to 120 individuals, compared to 144 possible relationships for modern humans. The interbreeding between Neanderthals, Denisovans, and modern humans was not all one way, and Sergi Castellano of the Max Planck Institute for Evolutionary Anthropology reported in 2016 that while Denisovan and Neanderthal genomes are more related to each other than they are to us, Siberian Neanderthal genomes show more similarity to modern human genes than do European Neanderthal populations. This suggests Neanderthal populations interbred with modern humans around 100,000 years ago, probably somewhere in the Near East. The genetic legacy of these lost cousins is still present in modern humans, and it has been shown that alleles thought to have originated in Neanderthals and Denisovans have been identified at several genetic loci in the genomes of modern humans outside Africa. The flow of genes from Neanderthal populations to modern humans was not all one way, and it has been shown that Neanderthal DNA segments may be associated with a higher rate of depression in modern humans. The story of the lost cousins is one of interbreeding, adaptation, and the complex web of relationships that have shaped our species over millions of years.

The Small Giants

Homo floresiensis, which lived from approximately 190,000 to 50,000 years before present, has been nicknamed the hobbit for its small size, possibly a result of insular dwarfism. The main find was a skeleton believed to be a woman of about 30 years of age, found in 2003, and it has been dated to approximately 18,000 years old. The living woman was estimated to be one meter in height, with a brain volume of just 380 cubic centimeters, considered small for a chimpanzee and less than a third of the Homo sapiens average of 1400 cubic centimeters. However, there is an ongoing debate over whether Homo floresiensis is indeed a separate species, and some scientists hold that it was a modern Homo sapiens with pathological dwarfism. This hypothesis is supported in part, because some modern humans who live on Flores, the Indonesian island where the skeleton was found, are pygmies, and pathological dwarfism could have resulted in a significantly diminutive human. The other major attack on Homo floresiensis as a separate species is that it was found with tools only associated with Homo sapiens, and the hypothesis of pathological dwarfism fails to explain additional anatomical features that are unlike those of modern humans but much like those of ancient members of our genus. Aside from cranial features, these features include the form of bones in the wrist, forearm, shoulder, knees, and feet, and this hypothesis fails to explain the find of multiple examples of individuals with these same characteristics, indicating they were common to a large population, and not limited to one individual. In 2016, fossil teeth and a partial jaw from hominins assumed to be ancestral to Homo floresiensis were discovered at Mata Menge, about 700,000 years ago, and they are noted by Australian archaeologist Gerrit van den Bergh for being even smaller than the later fossils. The existence of Homo floresiensis challenges our understanding of human evolution, and it suggests that the evolution of humans was not linear but weblike, with multiple species co-existing and evolving in different environments. The small size of Homo floresiensis was likely a result of insular dwarfism, a process that occurs when a population of animals is isolated on an island and evolves to become smaller over time. This process is thought to be a response to limited resources and the need to conserve energy, and it has been observed in many other species of animals, including elephants and hippos. The discovery of Homo floresiensis has also raised questions about the relationship between brain size and intelligence, and it suggests that small brains can still support complex behaviors and tool use. The story of the small giants is one of adaptation, survival, and the constant interplay between biological and cultural factors that have shaped our species over millions of years.

The Genetic Revolution

The genetic revolution in studies of human evolution started when Vincent Sarich and Allan Wilson measured the strength of immunological cross-reactions of blood serum albumin between pairs of creatures, including humans and African apes. The strength of the reaction could be expressed numerically as an immunological distance, which was in turn proportional to the number of amino acid differences between homologous proteins in different species. By constructing a calibration curve of the ID of species' pairs with known divergence times in the fossil record, the data could be used as a molecular clock to estimate the times of divergence of pairs with poorer or unknown fossil records. In their seminal 1967 paper in Science, Sarich and Wilson estimated the divergence time of humans and apes as four to five million years ago, at a time when standard interpretations of the fossil record gave this divergence as at least 10 to as much as 30 million years. Subsequent fossil discoveries, notably Lucy, and reinterpretation of older fossil materials, notably Ramapithecus, showed the younger estimates to be correct and validated the albumin method. Progress in DNA sequencing, specifically mitochondrial DNA and then Y-chromosome DNA, advanced the understanding of human origins, and the application of the molecular clock principle revolutionized the study of molecular evolution. On the basis of a separation from the orangutan between 10 and 20 million years ago, earlier studies of the molecular clock suggested that there were about 76 mutations per generation that were not inherited by human children from their parents, and this evidence supported the divergence time between hominins and chimpanzees noted above. However, a 2012 study in Iceland of 78 children and their parents suggests a mutation rate of only 36 mutations per generation, and this datum extends the separation between humans and chimpanzees to an earlier period greater than 7 million years ago. Additional research with 226 offspring of wild chimpanzee populations in eight locations suggests that chimpanzees reproduce at age 26.5 years on average, which suggests that the human divergence from chimpanzees occurred between 7 and 13 million years ago. And these data suggest that Ardipithecus, Orrorin, and Sahelanthropus all may be on the hominid lineage, and even that the separation may have occurred outside the East African Rift region. Furthermore, analysis of the two species' genes in 2006 provides evidence that after human ancestors had started to diverge from chimpanzees, interspecies mating between proto-human and proto-chimpanzees nonetheless occurred regularly enough to change certain genes in the new gene pool. A new comparison of the human and chimpanzee genomes suggests that after the two lineages separated, they may have begun interbreeding, and a principal finding is that the X chromosomes of humans and chimpanzees appear to have diverged about 1.2 million years more recently than the other chromosomes. The research suggests that there were in fact two splits between the human and chimpanzee lineages, with the first being followed by interbreeding between the two populations and then a second split. The suggestion of hybridization has startled paleoanthropologists, who nonetheless are treating the new genetic data seriously, and it has led to a reevaluation of the timeline of human evolution. The genetic revolution has provided new insights into the history of human evolution, and it has shown that the evolution of humans was not linear but weblike, with multiple species co-existing and evolving in different environments. The story of the genetic revolution is one of discovery, innovation, and the constant interplay between biological and cultural factors that have shaped our species over millions of years.

The Modern Mind

The transition to behavioral modernity has been characterized by some as a Great Leap Forward, or as the Upper Palaeolithic Revolution, due to the sudden appearance in the archaeological record of distinctive signs of modern behavior and big game hunting. Evidence of behavioral modernity significantly earlier also exists from Africa, with older evidence of abstract imagery, widened subsistence strategies, more sophisticated tools and weapons, and other modern behaviors, and many scholars have recently argued that the transition to modernity occurred sooner than previously believed. Other scholars consider the transition to have been more gradual, noting that some features had already appeared among archaic African Homo sapiens 300,000 to 200,000 years ago. Modern humans started burying their dead, making clothing from animal hides, hunting with more sophisticated techniques, such as using pit traps or driving animals off cliffs, and cave painting. As human culture advanced, different populations innovated existing technologies, and artifacts such as fish hooks, buttons, and bone needles show signs of cultural variation, which had not been seen prior to 50,000 years before present. Typically, the older Homo neanderthalensis populations did not vary in their technologies, although the Chatelperronian assemblages have been found to be Neanderthal imitations of Homo sapiens Aurignacian technologies. Recent evidence suggests that the Australian Aboriginal population separated from the African population 75,000 years ago, and that they made a sea journey 60,000 years ago, which may diminish the significance of the Upper Paleolithic Revolution. The transition to behavioral modernity was a complex process that involved the development of language, symbolic thinking, and complex social structures, and it was closely linked to the expansion of the human brain. The brain of a modern human is nearly three times the size of a chimpanzee or gorilla brain, and the evolution of the brain was closely linked to the development of tool use, language, and social behavior. The development of the brain was also linked to the development of cooking, which released increased nutritional value and allowed for the growth of larger brains. The control of fire and the use of cooking were key adaptations that separated Homo from tree-sleeping Australopithecines, and they were crucial for the survival and expansion of early human populations. The transition to behavioral modernity was also linked to the development of cultural evolution, which studies human sociocultural change from an evolutionary perspective. Cultural evolution has been shown to drive the development of new technologies and the expansion of human populations, and it has been argued that human evolution has accelerated since the development of agriculture 10,000 years ago and civilization some 5,000 years ago. The story of the modern mind is one of adaptation, innovation, and the constant interplay between biological and cultural factors that have shaped our species over millions of years.