Like every other species, humans have evolved a combination of characteristics that enabled them to survive and flourish. In this section we examine what characteristics are responsible for human success and how and when humans evolved.
Human Biological Characteristics
What makes humans unique as a species? Human success can be attributed in large part to our ability to perform complex reasoning, coupled with an exceptional ability to learn, to make and use sophisticated tools, and to communicate using complex language. These human abilities also required the evolution of at least three distinct physical characteristics. We have a very large brain relative to body size, our hands are capable of fine manipulation and coordination (Figure 1), and we walk upright (bipedal), freeing our hands to use tools.
Figure 1 Even young children can manipulate objects.
But dolphins have large brains, many animals can communicate, and apes and some birds make and use tools. What is different in humans is the degree to which these and other biological characteristics have evolved.
Human Phylogeny
Homo sapiens are primates. Primates are a relatively small group of mammals characterized by large brains relative to body size, forward-directed eyes, flexible hands and feet, and arms that can rotate fully. Many primates also have opposable thumbs that can touch their fingers and enable them to hold and manipulate objects. Most primates have tails (Figure 2).
primate: a group of relatively large-brained, mostly arboreal mammals that includes prosimians, monkeys, apes, and humans
Figure 2 Cladogram showing the phylogeny of primates
Image: A diagram showing the phylogeny of primates from the earliest primate from 60 mya, which today is the Prosimians, Anthropoids are New World monkeys and Old World monkeys, Hominoids are gibbons, orangutans, chimpanzees and gorillas, Hominids are humans.
All primates share a common ancestor dating from about 60 million to 70 million years ago. The evolutionary tree branches early on, giving rise to the prosimians, relatively small nocturnal species, and the anthropoids. The anthropoid group then splits into two distinct groups: the monkeys and the hominoids, or apes. During this
prosimian: the group of primates that includes lemurs, lorises, and tarsiers
anthropoid: the group of primates that includes monkeys, apes, and humans
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time, continental drift was separating South America (the New World) from the land mass that would form Africa and Eurasia (the Old World). With the split, the early ancestors of monkeys divided into New World and Old World populations. The apes, easily distinguished by their lack of a tail, branched into the lesser apes, the gibbons, and what are referred to as the great apes and humans.
Genetic sequences of the entire human and chimpanzee genomes have now been completed. Detailed analysis of the genomes confirms that chimpanzees are our closest living relatives. Humans and chimpanzees share approximately 98.8 % of their DNA. Humans and chimpanzees both differ from gorillas by about 1.6 %. The most recent common ancestor we share with chimpanzees lived in Africa more than 6 million years ago. The branch ending with modern humans is a Glade of many different species, including our direct ancestors. Members of this Glade are called hominids.
hominid: all species descended from the most recent common ancestor of chimpanzees and humans that are on the human side of the lineage
The Hominid Fossil Record
Hominid fossils record information about the sequence of steps in the evolution of humans. Fossils of hip bones, feet, leg bones, and footsteps provide information about whether or not a species walked upright. Fossils of the skull can be measured and used to track trends in brain size (Figure 3). Remains of stone tools and burial sites inform scientist about tool use and early human culture.
Figure 3 Fossil skulls of (a) Australopithecus africanus, (b) Homo habilis, and (c) Homo neanderthalensis show a progression in brain size. Note that bipedalism evolved in Australopithecus before the subsequent evolution of large brains.
Between 6 million and 7 million years ago, Sahelanthropus was beginning to occasionally walk upright (Figure 4(a), next page). Recently discovered fossils, and the first evidence of stone tools used by Australopithecus afarensis, date to 3.4 million years ago. By 2 million years ago, the first members of our own genus, Homo gautengensis, and Homo habilis had evolved. Homo habilis, often called the handy man, was making stone axes and large cutting tools. Tools for hunting permitted hominids to dramatically increase the amount of meat in their diet, providing a rich source of protein and fats. By this time, hominid brains were significantly larger than the chimpanzee-sized brains of australopithecines.
The use of hearths for cooking dates to at least 790 000 years ago, and fire may have been used for cooking as early as 1.5 million years ago. Cooking food may have reduced disease and increased the variety of foods that could be consumed. Perhaps most notably, hominid brain size increased relatively rapidly from about 800 000 years ago to 200 000 years ago. During that time, Homo heidelbergensis evolved and may have given rise to both Homo neanderthalensis and Homo sapiens. The first modern humans had evolved by about 100 000 years ago in East Africa. Figure 4(b) (next page) illustrates a simplified version of a widely accepted but tentative cladogram of the Homo genus.
Today the hominid fossil record of more than 20 species consists of fossils ranging in size from small bone fragments to almost complete skeletons. They include the spectacular Australopithecus afarensis fossil finds at Laetoli, Tanzania, which include a set of 69 footprints dated to 3.6 million years ago. These footprints show that human ancestors evolved the ability to walk upright long before they had large brains (Figure 5, next page). Although the precise relationships among the many early hominid species remain unclear, an early branch probably gave rise to a number of robust Paranthropus species with heavy jaws and relatively small brains, while another ultimately gave rise to the genus Homo.
LEARNING TIP
Becoming Human
Hominid evolution happened gradually over millions of years. Over time, an Australopithecus species changed enough to be considered the first Homo species. Although the process is continuous, scientists must still decide on a specific point to switch naming from one genus to another. This is analogous to becoming an adult. Even though the process is gradual, you gain adult status "instantly" on your eighteenth birthday.
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Figure 4 (a) Time frames during which early hominid species were living. Two very recently discovered hominid species, Homo gautengensis and Australopithecus sediba, are not included in this chart. (b) This simplified cladogram depicts the probable relationships within the Homo genus. Fossil evidence of the new species Homo gautengenis suggests it may have been an ancestor of Homo habilis.
Image: (a) a diagram to show which early hominid species were living. Possible bipedal locomotion, 7 mya Sahelanthropus tchadensis, 6 mya Orrorin tugenensis and Ardipithecus kadabba, 5 mya Ardipithecus ramidus, 4 mya Australopithecus anamensis. Then the diagram is cut in half by the Laetoli footprints and Bipedal locomotion 4 mya Kenyanthropus platyops and Australopithecus afarensis, 3 mya Paranthropus aethiopicus and Australopithecus africanus and Australopithecus garhi, 2 mya Paranthropus robustus and Paranthropus boisei and Homo rudolfensis and Homo habilis and Homo ergaster and Homo georgicus and Homo erectus, 1 mya Homo antecessor and Homo rhodesiensis and Homo pekinensis and Homo heidelbergensis and Homo neanderthalersis and Homo sapiens.
In diagram (b) this is a simplified version that starts with Australopithecus ancestor and at 3 mya is Homo gautengensis? And near 2 mya is Homo habilus and Homo genus, near 1 mya Homoerectus which lead to Homo neanderthalensis and Homo heidelbergensis and Homo sapiens.
Figure 5 The Laetoli footprint fossils are clear evidence of upright walking dating to about 3.6 million years ago.
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Out of Africa
Fossils record the history of the distribution of species in both time and space. Fossil evidence tells the story of when and where our ancestors evolved and how they spread out across and around the world. [Go To Nelson Science]
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Anthropologist
Anthropologists study the origins of humanity. To learn more about becoming an anthropologist,
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All early hominids evolved and lived in Africa (Figure 6). The first species to spread beyond Africa was Homo erectus, about 1.9 million years ago. The H. erectus population that left Africa spread out across much of Eurasia and survived until at least 100000 years ago. The next species to spread beyond Africa, some 500000 to 300000 years ago, were the ancestors of the Neanderthals. Homo neanderthalensis populated parts of Europe. Relatively soon after the earliest modern humans evolved, they too began to spread out of Africa and into Europe and Asia, eventually reaching the Americas. [Go To Nelson Science]
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Figure 6 Migration routes of early hominids
Image: A map of the world that depicts the early routes of travel by hominids
Fossils and DNA
In what can only be described as a stunning technological achievement, scientists have been able to extract enough DNA from Neanderthal bones to sequence the entire Neanderthal genome. This has allowed geneticists to compare the DNA of Neanderthals with that of modern humans. The most striking discovery is that some small sequences of the Neanderthal genome are found in humans of Asian and European descent. These genetic remnants no longer have any function in humans, but they do suggest that some interbreeding may have occurred when early Homo sapiens made contact with Neanderthals.
Cultural Evolution
The first biologists to compare human and chimpanzee DNA were so struck by their similarity that they joked that perhaps the only differences between humans and chimpanzees were cultural. Their joke was not without some grain of truth. While our genetic differences are less than 2 %, our cultural differences are enormous.
Like humans, chimpanzees engage in some ritualized behaviours and use symbolic gestures to communicate (Figure 7). They also have a complex social organization. However, neither the chimpanzee nor any other animal has developed anything comparable to the extraordinary richness of human culture. More than 6000 human languages have been spoken, and human societies have engaged in countless artistic endeavours in music, dance, and the fine arts. Humans admire and cherish the talents of others. We have athletic heroes and movie stars. Human societies have different rituals, customs, and belief systems.
Figure 7 Chimpanzees use both facial expressions and simple hand gestures to communicate.
It is fascinating to consider how biological evolution might have influenced the development of culture and vice versa. Evidence suggests that from the time of our common ancestor with the chimpanzee, our ancestors lived as hunter—gatherers for more than 300 000 generations. In only the last 1000 generations or less have humans domesticated
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plants and animals, developed agricultural systems, and begun to live in large population centres. Only during the last 10 generations has our population size skyrocketed.
There is evidence in the fossil record that our own species, Homo sapiens, and Homo neanderthalensis both performed burials and made body ornaments more than 50 000 years ago (Figure 8). While some believe that both species evolved these behaviours independently, others suggest that the Neanderthals began copying the activities of humans shortly after the two species came in contact.
Figure 8 (a) Neanderthals performed burial rituals. (b) Perforated shells made into a neck pendant along with pigments that might have been used as cosmetics—from a Neanderthal site
Image: Two pictures (a) shows a grave site with bones and the second picture shows (b) a cracked shell that may have been used as a necklace.
UNIT TASK BOOKMARK
As you work on your Unit Task, consider how changes in our culture influence the evolution of other species.
A vital component of human culture is language and communication. Without the development of a rich spoken language, humans could not have realized our current success as a species. Evolutionary biologists are just beginning to understand the evolution of speech. We now know, for example, that a gene called FoxP2 codes for a protein that regulates a number of other genes and is vital for human speech. Individuals with even one defective copy of the gene have a severe speech and language disorder. The gene is found in all mammals and is highly conserved—meaning it varies little from species to species. The human and Neanderthal version of the FoxP2 gene differs from that of the chimpanzee in just two bases out of more than 2100. Genetic analysis also provides evidence that these two mutations have been strongly favoured by natural selection. It could be that one or both of these mutations provided our early ancestors with an enhanced ability to communicate.
Cultural Evolution Influences Biological Evolution
An interesting example of how cultural evolution influences biological evolution is evident in the recent evolution of lactose tolerance. Lactose tolerance evolved as human populations began to domesticate goats, cattle, and camels, and consume their milk. In these populations natural selection favoured those rare individuals who were more capable of digesting lactose. Mathematical modelling suggests that tolerant individuals had a 4 % to 10 % enhanced reproductive success in these populations. In this way, it was the cultural choice to domesticate livestock and consume their milk that created the new selective pressure for the evolution of lactose tolerance (Figure 9). In fact, the very rapid changes in human culture over the past 10 000 years have actually sped up human evolution. Researchers examining the human genome have uncovered evidence to suggest that the rate of human evolution has been greater over the past several thousand years than during the past few million years.
Figure 9 Lactose tolerance is a recently evolved trait.
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Human culture has influenced the evolution of many other species but none more so than domesticated species. Indeed, the domestication of plants and animals by artificial selection and breeding is nothing less than directed evolution by humans. It is interesting to consider that after 300 000 generations of our own biological evolution, the human population numbered no more than a few million. It has been our ability to direct the evolution of other species that has allowed us to mass-produce foods and feed our own population that now numbers in the billions (Figure 10).
Figure 10 Humans have directly influenced the evolution of many species, including species we have domesticated through artificial selection. "ya" means years ago.
Image: An illustration of a map with locations near the Mediterranean Sea and how the evolution of species of cattle 10 000 ya, goats 11 000 ya, pigs 10 500 ya, sheep 11 000 ya, emmer wheat 10 500 ya and barley 12 000 ya happened.
Human Races
Detailed comparisons of human populations from around the world have conclusively shown that, from a biological perspective, human "races" do not exist. Traits that we associate with races, most notably skin colour, are visually obvious but genetically minimal. There is far more diversity within so-called races than there is between races. The classification of humans by race is therefore a cultural choice on a par with classifying people based the language they speak, their blood type, or their religious beliefs (Figure 11).
Figure 11 Although humans have used skin colour as a way of categorizing people into races, the underlying genetic differences are very slight and there are no biologically distinct races of humans.
Are Humans Still Evolving?
Is our evolutionary path of any consequence today? Does it matter if we evolved from a common ancestor of apes? Are we still evolving today? Understanding our evolutionary past and how evolutionary processes work today has many benefits. For example, it can provide insights into healthcare problems. We know that in our past, rich food sources were scarce and natural selection favoured individuals who could detect sweet-tasting and fatty foods and could gain weight during times when food was readily available. Our sense of taste evolved to "let us know" that these were valuable foods. In our modern world, we still relish these foods but now it is easy for us to overindulge (Figure 12). This evolutionary preference for ice cream and similar foods is therefore partly responsible for the problems of obesity and heart disease.
Figure 12 Throughout much of our evolutionary past, natural selection favoured a fondness for rich foods.
Darwinian medicine, the use of evolutionary theory to understand medicine, is providing many important insights into the cause and spread of diseases. An evolutionary perspective can be very important. For example, humans often get a fever with a serious infection. The first question an evolutionary biologist might ask is whether or not a fever is an evolved response of the body to help fight off the infection. If this is the case, then taking a drug to lower your body temperature might be unwise. Similarly, is coughing an evolved mechanism to expel disease-causing organisms from your lungs? If so, would taking a medication that suppresses a cough be helpful?
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In modern society, vitamin D deficiency is widespread. This is not at all surprising from an evolutionary perspective. For millions of years, ancestral and modern humans spent time outdoors every day and evolved the ability to synthesize vitamin D when exposed to sunlight. Today many people spend little or no time outside, and
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when they do they are told to use sunscreen. Researchers are investigating the best way to balance the need to protect ourselves from damaging UV radiation and the risk of skin cancer with our evolved requirement to synthesize vitamin D.
In serious infectious diseases, biologists can track the evolution of the disease-causing agents as well as human evolutionary responses to these diseases. For example, a newly discovered allele called CCR5-D32 provides very strong protection against HIV/AIDS infections and may be under the influence of strong selective pressure in some populations. [Go To Nelson Science]
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Epidemiologist
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A Human Legacy?
Evolution can inform us about where we have come from but not where we are going. We have evolved a brain and body capable of great achievements in the arts and in understanding the world around us. We have discovered our own biological origins and have the tools and freedom to direct our own future. Yet humans also behave in ways that threaten our own future and the future of other species. [Go To Nelson Science]
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Behavioural Psychologist
Behavioural psychologists study human behaviour. To learn more about becoming a behavioural psychologist,
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There is no doubt that humans are biologically limited by our evolutionary history. We are not going to sprout wings and fly, but there is nothing in our genes that prevents us from making intelligent choices about our own future. We have certainly made much progress—legalized slavery and routine sacrifice were once widespread, accepted practices. Today, in most countries women and men have equal rights under the law, and forced child labour has been criminalized. We have evolved a brain capable of knowing which actions are needed to ensure a secure and sustainable future for ourselves and our descendants as well as for the biodiversity of life on Earth as a whole. How human society will decide to act in the future remains an unknown.
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8.7 Summary
- Key evolutionary characteristics of humans include bipedalism, large brain size, and hands capable of fine manipulation.
- Chimpanzees are our most closely related living species. - There is a rich fossil record of human ancestors consisting of more than 20 hominid species.
- Homo sapiens first evolved in Africa and began spreading out from Africa about 50 000 years ago.
- Biological and cultural evolution influence each other.
- An understanding of human evolution has many applications.
8.7 Questions
1. What selective advantage does each of the following traits provide to humans? K/U (a) a large brain (b) upright walking (c) complex finger movements (d) complex language
4. What evidence suggests that human ancestors walked upright before they evolved large brains? T/I A
5. Humans have an innate ability to leam new behaviours. In contrast, most behaviours of most species are instinctive. This suggests that there are advantages and disadvantages of learned behaviour. For each of the following, decide how the learned behaviour is advantageous and/or disadvantageous: T/U (a) Young humans learn to walk at about one year old, while deer fawns walk when they are a few hours old. (b) Young humans learn their parents' language, while most frogs and birds sing their mating calls instinctively. (c) Humans learn to adapt to many different environmental conditions, while most species use instinctive behaviours suited to a specific type of environment.
9. Humans have lived as small groups of hunter—gatherers during most of the last 100 000 years. Our food consisted of some animals and many plants. Sweet foods, fatty foods, and salts were relatively rare. Today the favourite foods of most people fit into these same categories. T/I A (a) Make a list of 10 of your favourite foods. How many of them are sweet, high in fat, or salty? (b) Suggest a way that natural selection might have caused us to evolve love for these foods. (c) How has the evolutionary love of these foods influenced societal problems related to obesity and poor eating habits?
11. Homo sapiens have existed for only a few hundred thousand years. Most of the recent success of humans can be attributed to cultural and technological advances. T/I A (a) Do you think humans will continue to evolve as a species? Support your ideas. (b) What selective pressures do you think humans may be experiencing now and may experience in the future? (c) Hypothesize about potential human adaptations that could result.
Like every other species, humans have evolved a combination of characteristics that enabled them to survive and flourish. In this section we examine what characteristics are responsible for human success and how and when humans evolved.
Human Biological Characteristics
What makes humans unique as a species? Human success can be attributed in large part to our ability to perform complex reasoning, coupled with an exceptional ability to learn, to make and use sophisticated tools, and to communicate using complex language. These human abilities also required the evolution of at least three distinct physical characteristics. We have a very large brain relative to body size, our hands are capable of fine manipulation and coordination (Figure 1), and we walk upright (bipedal), freeing our hands to use tools.
Figure 1 Even young children can manipulate objects.
But dolphins have large brains, many animals can communicate, and apes and some birds make and use tools. What is different in humans is the degree to which these and other biological characteristics have evolved.
Human Phylogeny
Homo sapiens are primates. Primates are a relatively small group of mammals characterized by large brains relative to body size, forward-directed eyes, flexible hands and feet, and arms that can rotate fully. Many primates also have opposable thumbs that can touch their fingers and enable them to hold and manipulate objects. Most primates have tails (Figure 2).
primate: a group of relatively large-brained, mostly arboreal mammals that includes prosimians, monkeys, apes, and humans
Figure 2 Cladogram showing the phylogeny of primates
Image: A diagram showing the phylogeny of primates from the earliest primate from 60 mya, which today is the Prosimians, Anthropoids are New World monkeys and Old World monkeys, Hominoids are gibbons, orangutans, chimpanzees and gorillas, Hominids are humans.
All primates share a common ancestor dating from about 60 million to 70 million years ago. The evolutionary tree branches early on, giving rise to the prosimians, relatively small nocturnal species, and the anthropoids. The anthropoid group then splits into two distinct groups: the monkeys and the hominoids, or apes. During this
prosimian: the group of primates that includes lemurs, lorises, and tarsiers
anthropoid: the group of primates that includes monkeys, apes, and humans
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time, continental drift was separating South America (the New World) from the land mass that would form Africa and Eurasia (the Old World). With the split, the early ancestors of monkeys divided into New World and Old World populations. The apes, easily distinguished by their lack of a tail, branched into the lesser apes, the gibbons, and what are referred to as the great apes and humans.
Genetic sequences of the entire human and chimpanzee genomes have now been completed. Detailed analysis of the genomes confirms that chimpanzees are our closest living relatives. Humans and chimpanzees share approximately 98.8 % of their DNA. Humans and chimpanzees both differ from gorillas by about 1.6 %. The most recent common ancestor we share with chimpanzees lived in Africa more than 6 million years ago. The branch ending with modern humans is a Glade of many different species, including our direct ancestors. Members of this Glade are called hominids.
hominid: all species descended from the most recent common ancestor of chimpanzees and humans that are on the human side of the lineage
The Hominid Fossil Record
Hominid fossils record information about the sequence of steps in the evolution of humans. Fossils of hip bones, feet, leg bones, and footsteps provide information about whether or not a species walked upright. Fossils of the skull can be measured and used to track trends in brain size (Figure 3). Remains of stone tools and burial sites inform scientist about tool use and early human culture.
Figure 3 Fossil skulls of (a) Australopithecus africanus, (b) Homo habilis, and (c) Homo neanderthalensis show a progression in brain size. Note that bipedalism evolved in Australopithecus before the subsequent evolution of large brains.
Between 6 million and 7 million years ago, Sahelanthropus was beginning to occasionally walk upright (Figure 4(a), next page). Recently discovered fossils, and the first evidence of stone tools used by Australopithecus afarensis, date to 3.4 million years ago. By 2 million years ago, the first members of our own genus, Homo gautengensis, and Homo habilis had evolved. Homo habilis, often called the handy man, was making stone axes and large cutting tools. Tools for hunting permitted hominids to dramatically increase the amount of meat in their diet, providing a rich source of protein and fats. By this time, hominid brains were significantly larger than the chimpanzee-sized brains of australopithecines.
The use of hearths for cooking dates to at least 790 000 years ago, and fire may have been used for cooking as early as 1.5 million years ago. Cooking food may have reduced disease and increased the variety of foods that could be consumed. Perhaps most notably, hominid brain size increased relatively rapidly from about 800 000 years ago to 200 000 years ago. During that time, Homo heidelbergensis evolved and may have given rise to both Homo neanderthalensis and Homo sapiens. The first modern humans had evolved by about 100 000 years ago in East Africa. Figure 4(b) (next page) illustrates a simplified version of a widely accepted but tentative cladogram of the Homo genus.
Today the hominid fossil record of more than 20 species consists of fossils ranging in size from small bone fragments to almost complete skeletons. They include the spectacular Australopithecus afarensis fossil finds at Laetoli, Tanzania, which include a set of 69 footprints dated to 3.6 million years ago. These footprints show that human ancestors evolved the ability to walk upright long before they had large brains (Figure 5, next page). Although the precise relationships among the many early hominid species remain unclear, an early branch probably gave rise to a number of robust Paranthropus species with heavy jaws and relatively small brains, while another ultimately gave rise to the genus Homo.
LEARNING TIP
Becoming Human
Hominid evolution happened gradually over millions of years. Over time, an Australopithecus species changed enough to be considered the first Homo species. Although the process is continuous, scientists must still decide on a specific point to switch naming from one genus to another. This is analogous to becoming an adult. Even though the process is gradual, you gain adult status "instantly" on your eighteenth birthday.
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Figure 4 (a) Time frames during which early hominid species were living. Two very recently discovered hominid species, Homo gautengensis and Australopithecus sediba, are not included in this chart.
(b) This simplified cladogram depicts the probable relationships within the Homo genus. Fossil evidence of the new species Homo gautengenis suggests it may have been an ancestor of Homo habilis.
Image: (a) a diagram to show which early hominid species were living. Possible bipedal locomotion, 7 mya Sahelanthropus tchadensis, 6 mya Orrorin tugenensis and Ardipithecus kadabba, 5 mya Ardipithecus ramidus, 4 mya Australopithecus anamensis. Then the diagram is cut in half by the Laetoli footprints and Bipedal locomotion 4 mya Kenyanthropus platyops and Australopithecus afarensis, 3 mya Paranthropus aethiopicus and Australopithecus africanus and Australopithecus garhi, 2 mya Paranthropus robustus and Paranthropus boisei and Homo rudolfensis and Homo habilis and Homo ergaster and Homo georgicus and Homo erectus, 1 mya Homo antecessor and Homo rhodesiensis and Homo pekinensis and Homo heidelbergensis and Homo neanderthalersis and Homo sapiens.
In diagram (b) this is a simplified version that starts with Australopithecus ancestor and at 3 mya is Homo gautengensis? And near 2 mya is Homo habilus and Homo genus, near 1 mya Homoerectus which lead to Homo neanderthalensis and Homo heidelbergensis and Homo sapiens.
Figure 5 The Laetoli footprint fossils are clear evidence of upright walking dating to about 3.6 million years ago.
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Out of Africa
Fossils record the history of the distribution of species in both time and space. Fossil evidence tells the story of when and where our ancestors evolved and how they spread out across and around the world. [Go To Nelson Science]
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Anthropologist
Anthropologists study the origins of humanity. To learn more about becoming an anthropologist,
GO TO NELSON SCIENCE
All early hominids evolved and lived in Africa (Figure 6). The first species to spread beyond Africa was Homo erectus, about 1.9 million years ago. The H. erectus population that left Africa spread out across much of Eurasia and survived until at least 100000 years ago. The next species to spread beyond Africa, some 500000 to 300000 years ago, were the ancestors of the Neanderthals. Homo neanderthalensis populated parts of Europe. Relatively soon after the earliest modern humans evolved, they too began to spread out of Africa and into Europe and Asia, eventually reaching the Americas. [Go To Nelson Science]
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Figure 6 Migration routes of early hominids
Image: A map of the world that depicts the early routes of travel by hominids
Fossils and DNA
In what can only be described as a stunning technological achievement, scientists have been able to extract enough DNA from Neanderthal bones to sequence the entire Neanderthal genome. This has allowed geneticists to compare the DNA of Neanderthals with that of modern humans. The most striking discovery is that some small sequences of the Neanderthal genome are found in humans of Asian and European descent. These genetic remnants no longer have any function in humans, but they do suggest that some interbreeding may have occurred when early Homo sapiens made contact with Neanderthals.
Cultural Evolution
The first biologists to compare human and chimpanzee DNA were so struck by their similarity that they joked that perhaps the only differences between humans and chimpanzees were cultural. Their joke was not without some grain of truth. While our genetic differences are less than 2 %, our cultural differences are enormous.
Like humans, chimpanzees engage in some ritualized behaviours and use symbolic gestures to communicate (Figure 7). They also have a complex social organization. However, neither the chimpanzee nor any other animal has developed anything comparable to the extraordinary richness of human culture. More than 6000 human languages have been spoken, and human societies have engaged in countless artistic endeavours in music, dance, and the fine arts. Humans admire and cherish the talents of others. We have athletic heroes and movie stars. Human societies have different rituals, customs, and belief systems.
Figure 7 Chimpanzees use both facial expressions and simple hand gestures to communicate.
It is fascinating to consider how biological evolution might have influenced the development of culture and vice versa. Evidence suggests that from the time of our common ancestor with the chimpanzee, our ancestors lived as hunter—gatherers for more than 300 000 generations. In only the last 1000 generations or less have humans domesticated
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plants and animals, developed agricultural systems, and begun to live in large population centres. Only during the last 10 generations has our population size skyrocketed.
There is evidence in the fossil record that our own species, Homo sapiens, and Homo neanderthalensis both performed burials and made body ornaments more than 50 000 years ago (Figure 8). While some believe that both species evolved these behaviours independently, others suggest that the Neanderthals began copying the activities of humans shortly after the two species came in contact.
Figure 8 (a) Neanderthals performed burial rituals. (b) Perforated shells made into a neck pendant along with pigments that might have been used as cosmetics—from a Neanderthal site
Image: Two pictures (a) shows a grave site with bones and the second picture shows (b) a cracked shell that may have been used as a necklace.
UNIT TASK BOOKMARK
As you work on your Unit Task, consider how changes in our culture influence the evolution of other species.
A vital component of human culture is language and communication. Without the development of a rich spoken language, humans could not have realized our current success as a species. Evolutionary biologists are just beginning to understand the evolution of speech. We now know, for example, that a gene called FoxP2 codes for a protein that regulates a number of other genes and is vital for human speech. Individuals with even one defective copy of the gene have a severe speech and language disorder. The gene is found in all mammals and is highly conserved—meaning it varies little from species to species. The human and Neanderthal version of the FoxP2 gene differs from that of the chimpanzee in just two bases out of more than 2100. Genetic analysis also provides evidence that these two mutations have been strongly favoured by natural selection. It could be that one or both of these mutations provided our early ancestors with an enhanced ability to communicate.
Cultural Evolution Influences Biological Evolution
An interesting example of how cultural evolution influences biological evolution is evident in the recent evolution of lactose tolerance. Lactose tolerance evolved as human populations began to domesticate goats, cattle, and camels, and consume their milk. In these populations natural selection favoured those rare individuals who were more capable of digesting lactose. Mathematical modelling suggests that tolerant individuals had a 4 % to 10 % enhanced reproductive success in these populations. In this way, it was the cultural choice to domesticate livestock and consume their milk that created the new selective pressure for the evolution of lactose tolerance (Figure 9). In fact, the very rapid changes in human culture over the past 10 000 years have actually sped up human evolution. Researchers examining the human genome have uncovered evidence to suggest that the rate of human evolution has been greater over the past several thousand years than during the past few million years.
Figure 9 Lactose tolerance is a recently evolved trait.
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Human culture has influenced the evolution of many other species but none more so than domesticated species. Indeed, the domestication of plants and animals by artificial selection and breeding is nothing less than directed evolution by humans. It is interesting to consider that after 300 000 generations of our own biological evolution, the human population numbered no more than a few million. It has been our ability to direct the evolution of other species that has allowed us to mass-produce foods and feed our own population that now numbers in the billions (Figure 10).
Figure 10 Humans have directly influenced the evolution of many species, including species we have domesticated through artificial selection. "ya" means years ago.
Image: An illustration of a map with locations near the Mediterranean Sea and how the evolution of species of cattle 10 000 ya, goats 11 000 ya, pigs 10 500 ya, sheep 11 000 ya, emmer wheat 10 500 ya and barley 12 000 ya happened.
Human Races
Detailed comparisons of human populations from around the world have conclusively shown that, from a biological perspective, human "races" do not exist. Traits that we associate with races, most notably skin colour, are visually obvious but genetically minimal. There is far more diversity within so-called races than there is between races. The classification of humans by race is therefore a cultural choice on a par with classifying people based the language they speak, their blood type, or their religious beliefs (Figure 11).
Figure 11 Although humans have used skin colour as a way of categorizing people into races, the underlying genetic differences are very slight and there are no biologically distinct races of humans.
Are Humans Still Evolving?
Is our evolutionary path of any consequence today? Does it matter if we evolved from a common ancestor of apes? Are we still evolving today? Understanding our evolutionary past and how evolutionary processes work today has many benefits. For example, it can provide insights into healthcare problems. We know that in our past, rich food sources were scarce and natural selection favoured individuals who could detect sweet-tasting and fatty foods and could gain weight during times when food was readily available. Our sense of taste evolved to "let us know" that these were valuable foods. In our modern world, we still relish these foods but now it is easy for us to overindulge (Figure 12). This evolutionary preference for ice cream and similar foods is therefore partly responsible for the problems of obesity and heart disease.
Figure 12 Throughout much of our evolutionary past, natural selection favoured a fondness for rich foods.
Darwinian medicine, the use of evolutionary theory to understand medicine, is providing many important insights into the cause and spread of diseases. An evolutionary perspective can be very important. For example, humans often get a fever with a serious infection. The first question an evolutionary biologist might ask is whether or not a fever is an evolved response of the body to help fight off the infection. If this is the case, then taking a drug to lower your body temperature might be unwise. Similarly, is coughing an evolved mechanism to expel disease-causing organisms from your lungs? If so, would taking a medication that suppresses a cough be helpful?
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In modern society, vitamin D deficiency is widespread. This is not at all surprising from an evolutionary perspective. For millions of years, ancestral and modern humans spent time outdoors every day and evolved the ability to synthesize vitamin D when exposed to sunlight. Today many people spend little or no time outside, and
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when they do they are told to use sunscreen. Researchers are investigating the best way to balance the need to protect ourselves from damaging UV radiation and the risk of skin cancer with our evolved requirement to synthesize vitamin D.
In serious infectious diseases, biologists can track the evolution of the disease-causing agents as well as human evolutionary responses to these diseases. For example, a newly discovered allele called CCR5-D32 provides very strong protection against HIV/AIDS infections and may be under the influence of strong selective pressure in some populations. [Go To Nelson Science]
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Epidemiologist
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A Human Legacy?
Evolution can inform us about where we have come from but not where we are going. We have evolved a brain and body capable of great achievements in the arts and in understanding the world around us. We have discovered our own biological origins and have the tools and freedom to direct our own future. Yet humans also behave in ways that threaten our own future and the future of other species. [Go To Nelson Science]
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Behavioural Psychologist
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There is no doubt that humans are biologically limited by our evolutionary history. We are not going to sprout wings and fly, but there is nothing in our genes that prevents us from making intelligent choices about our own future. We have certainly made much progress—legalized slavery and routine sacrifice were once widespread, accepted practices. Today, in most countries women and men have equal rights under the law, and forced child labour has been criminalized. We have evolved a brain capable of knowing which actions are needed to ensure a secure and sustainable future for ourselves and our descendants as well as for the biodiversity of life on Earth as a whole. How human society will decide to act in the future remains an unknown.
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8.7 Summary
- Key evolutionary characteristics of humans include bipedalism, large brain size, and hands capable of fine manipulation.
- Chimpanzees are our most closely related living species.
- There is a rich fossil record of human ancestors consisting of more than 20 hominid species.
- Homo sapiens first evolved in Africa and began spreading out from Africa about 50 000 years ago.
- Biological and cultural evolution influence each other.
- An understanding of human evolution has many applications.
8.7 Questions
1. What selective advantage does each of the following traits provide to humans? K/U
(a) a large brain
(b) upright walking
(c) complex finger movements
(d) complex language
4. What evidence suggests that human ancestors walked upright before they evolved large brains? T/I A
5. Humans have an innate ability to leam new behaviours. In contrast, most behaviours of most species are instinctive. This suggests that there are advantages and disadvantages of learned behaviour. For each of the following, decide how the learned behaviour is advantageous and/or disadvantageous: T/U
(a) Young humans learn to walk at about one year old, while deer fawns walk when they are a few hours old.
(b) Young humans learn their parents' language, while most frogs and birds sing their mating calls instinctively.
(c) Humans learn to adapt to many different environmental conditions, while most species use instinctive behaviours suited to a specific type of environment.
9. Humans have lived as small groups of hunter—gatherers during most of the last 100 000 years. Our food consisted of some animals and many plants. Sweet foods, fatty foods, and salts were relatively rare. Today the favourite foods of most people fit into these same categories. T/I A
(a) Make a list of 10 of your favourite foods. How many of them are sweet, high in fat, or salty?
(b) Suggest a way that natural selection might have caused us to evolve love for these foods.
(c) How has the evolutionary love of these foods influenced societal problems related to obesity and poor eating habits?
11. Homo sapiens have existed for only a few hundred thousand years. Most of the recent success of humans can be attributed to cultural and technological advances. T/I A
(a) Do you think humans will continue to evolve as a species? Support your ideas.
(b) What selective pressures do you think humans may be experiencing now and may experience in the future?
(c) Hypothesize about potential human adaptations that could result.