Chapter 6 Outline

Introduction

• The discovery in Africa in 2010 of a new and previously unknown species of early human ancestors, Australopithecus sediba, led to the further discovery of thousands of new fossils including numerous intact individuals exhibiting important differences from A. sediba, subsequently named Homo naledi.
• Many details remain ambiguous about these early human ancestors, including their geographic range and their relationship with each other. But both exhibit a mix of physical traits with similarities to two known genera of human ancestors.
• The significance of these finds is that they both represent new branches in our family tree, affecting understanding of our past.
• Fossils are scarce, and complete ones which allow identification of a new species of early human ancestors is rare. This is the work of paleoanthropologists: physical anthropologists and archaeologists who study the fossilized remains of ancient hominids to shed light on their biological and behavioral evolution.
• Advancements in genomic science and technology, including the use of DNA evidence recovered from fossils in the lab, have also had a major influence on this field. Such technology was a key element in the discovery of a new group of 40,000 year-old humans called the Denisovans. A transnational scientific initiative called the Neanderthal Genome Project studied the Neanderthals by mapping the genome: the complete set of an organism’s DNA.
• At the heart of anthropology’s interest in our human ancestors is a key question: when, where, and how did our human ancestors emerge, and under what conditions did modern humans evolve? Embedded within this larger question are a number of more focused problems:
• Who are our earliest possible ancestors?
• What did walking on two legs and having big brains mean for the early humans?
• Who were the first humans and where did they live?
• How do we know if the first humans were cultural beings, and what role did culture play in their evolution?
• The fossil record shows that the genus to which we belong, Homo, emerged some two million years ago and began to diversify between 1.8 million and 300,000 years ago, exhibiting a growing “humanness” in our ancestors.

Who Are Our Earliest Possible Ancestors?

• To answer this question, we must address two issues: our evolutionary relationship to the other apes, and how that relationship affects how we view and name certain fossils found from 6 to 1 mya: million years ago.
• Current knowledge of both morphological and molecular relationships among living apes and humans indicates that chimpanzees, gorillas, and humans are actually more similar to one another—and thus more closely related—than any of them is to the orangutan. This affects how paleoanthropologists reconstruct the evolutionary trajectory of our early human ancestors.
• All great apes and humans are placed together in the family Hominidae: a family of primates that includes the Hominids, namely humans and their ancestors.
• Within the Hominidae is the subfamily Homininae: the African subfamily of the Family Hominidae, which includes humans, chimpanzees and gorillas.
• Finally, we have the subfamily Ponginae: the Asian derived subfamily of Hominidae to which the Orangutan belongs.
• Members of our species are classed as representatives of the tribe Hominini: the tribe to which humans and our direct human ancestors belong, who are referred to as hominins.
• In this classification, all hominins are hominines, but the reverse is not true: only some hominines are hominins, these being modern humans and our direct lineage.
• The hominins share in common several unique traits which include:
• modifications in the lower body, the upper arms, and the backbone that make them capable of bipedal locomotion: the use of two legs rather than four for movement.
• smaller canine teeth than other members of the Hominidae family
• to help support bipedalism, a forward-placed foramen magnum: the opening at the base of the skull (cranium) where the spinal column enters and connects to the brain.
• a reduced Canine/Premolar-3 shearing complex: a condition in which the lower first premolar tooth is somewhat sharpened or flattened from rubbing against the upper canine as the mouth closes.
• Fossil evidence of ancestral hominins comes from Africa during the end of Miocene epoch (22 to 5.3 mya). Primitive hominin-like primates emerged in East Africa and in the Mediterranean region near North Africa, a pattern suggesting that the transition by one or more very old hominoid lineages toward a hominin form took place in these two areas.
• Debate persists whether early fossils are undoubtedly hominin. The study of morphological differences in fossils often results in taxonomic designations through the differences. Living ape species often exhibit peculiar anatomical quirks which are simply normal variations within a species. This sparks debate about whether or not morphological differences are as meaningful as they seem. This is important to keep in mind because it is not unique to these transitional hominines but to the hominins as well.
• Numerous identifiable hominins emerged during the Pliocene epoch (5.3-2.5 mya) and the Pleistocene (2.5 mya to 11,500 years ago). The evolutionary relationships between these and earlier Miocene hominoids remains unclear. The Plio-Pleistocene hominins are divided into three genera: Australopithecus, Paranthropus, and Homo. Numerous species of these genera overlapped in time, and in some cases geography, suggesting that there were multiple forms of co-existing hominins.
• The genus Australopithecus (“Southern Ape,” as many have been found in South Africa) has the longest presence in the fossil record, a span of over 2 million years. Originally discovered in the 1920s and found distributed widely throughout eastern and southern Africa, we have more information about these ancient hominins than we have about any other early hominin. Most researchers hypothesize that the human lineage emerged from the australopithecines: a word that refers to the genus Australopithecus.
• Australopithecines were between 1.2-1.4m (3 ft 11 in-4ft 7 in) tall, exhibited a fairly high degree of sexual dimorphism, with males larger than females, and were gracile: a body of slender-build.
• Their relatively large brain sizes, while still only 35% of the brain size of modern humans, distinguish them from their primate ancestors. They also had a gripping hand. There is some evidence to suggest that at least several members of this genus were processing food, possibly as early as 3.3 million years ago.
• Arm length suggests an at least partially arboreal existence, although they also had bipedal stature which allowed them to walk on two legs.
• Between about 2.7 and 1 mya, we find a cluster of hominin fossils that differ in morphology from the australopithecines. There are three known species in this genus, Paranthropus, which means “beside” or “near” humans.
• Paranthropines had larger brains than the australopithecines. They had broad, “dish-shaped” faces, almost small foreheads, widely flared cheekbones, a pronounced sagittal crest: a ridge running along the top of the cranium, usually representing increased bone area for the attachment of chewing muscles.
• They also had a forward jutting jaw and exhibit megadontia: the characteristic of having large molar teeth relative to body size.
• They may have relied more heavily on plant foods than australopithecines. They were ranged from 1.3-1.4m in height, and all three species were bipedal.
• They probably lived in open woodland or savannah landscapes and were almost certainly tool users.
• The genus Homo emerged out of one of the australopithecine lineages sometime between 3 and 2 million years ago. Most hominin fossils found in Africa and Asia dating to younger than about 1.8 mya are generally considered members of this genus. Disagreement persists about how many species they actually represent, or even if some early cases are members of the genus Homo or the genus Australopithecus.
• To be placed definitively in the genus Homo, fossils must display certain characteristics, a primary one being large cranial capacity.
• Early members of Homo were also competent bipeds, who probably lived in mixed savannah-woodlands landscapes. In addition, their fingers are slightly curved and strongly built, and their hands suggest the ability to use a precision grip, an important requirement for making tools. They made and used stone tools, called Olduwan tools: rocks that were modified to produce sharp flakes and edged
choppers.
• Both species of early Homo display a mix of ancestral and derived (unique) characteristics, and these are expressed differently in each species.
• Our very earliest ancestors were bipedal hominins living in Eastern and Southern Africa by at least 4 million years ago and probably earlier. They did not yet have the big brain that characterizes modern humans, but what we do see aligns with what we would expect our early ancestors would look like.
• Remaining unanswered is the question of which one is most directly related to us. Is one of these the “missing link?”
• The idea of a “missing link” has long captured popular imagination, suggesting that once we identify that link a lot of uncertainties about our past will fall into place. While paleoanthropologists agree that we do have a common ancestor, deciding which one is a major challenge, if not impossible, because the fossil record is still so incomplete. Evolutionary theory rejects the possibility of a “link” because that metaphor assumes a straight and linear series of relationships, which is not really how species evolve. A missing link will never be found. What we do have are many “missing links,” and what paleoanthropologists are looking for are evolutionary relationships between existing hominins.
• There is reasonable certainty that by 2 mya our own lineage had arisen, and our genus Homo probably emerged from Australopithecus lineage. Similarities in anatomy indicate that most, if not all, hominins from ~3 million years ago on are derived from the A. afarensis lineage.
• Dating issues with A. africanus as well as the existence and classification of Kenyanthropus and A. deyiremeda do pose some problems for claiming A. afarensis as our direct ancestor.
• Paranthropus was perhaps a sister genus that co-existed with our own for almost 1.5 million years.
• Understanding why Homo flourished as Paranthropus went extinct would afford us valuable insight into what it means to be human.
• We still do not know which of the early members of the genus Homo is our most direct ancestor.
• The present fossil evidence offers no completely right answers about the specific relationships among early hominins, though several possible phylogenies have been proposed, some supported by more evidence than others. As the fossil record grows, some of these phylogenetic ambiguities may clear up. At this stage, we focus on what two defining features of our lineage—bipedalism and big brains—mean for hominin evolution.

What Did Walking On Two Legs And Having Big Brains Mean For The Early Hominins?

• Bipedalism is one of the determining traits of the hominins and is directly linked with our emergence and our separation from the apes. Increased brain size is also significant as it has enabled us to acquire a degree of social complexity and tool use not seen in other apes.
• Australopithecines and Homo habilis anatomy would have allowed movement through the trees as well as bipedally on the ground. There is a difference between a bipedal- allowing anatomy that still enables the use of four limbs for movement and climbing and a bipedal-enforcing anatomy, as modern human have. We are still trying to understand why full-time bipedalism evolved.
• Currently, bipedalism is seen as a consequence of multiple, independent selections that have some relation to the following points:
• It aids carrying objects. Because it frees the arms, objects can be carried. These benefits would only be realized after bipedalism evolved, so it can’t be the reason bipedalism arose.
• It benefits hunting. Bipedalism is efficient for long-distance locomotion and enables weapon carrying. Archaeological evidence for hunting comes later than the development of bipedalism.
• It favors upright reaching. Changes in the upper and lower body enabling bipedalism enhanced the ability to reach for hanging fruit and plants. Erect posture benefits both arboreal or terrestrial forms, and so this also doesn’t explain why it arose.
• It aids vigilance and visual surveillance. The ability to stand on two legs gives a better view of potential predators. Because it aids predator avoidance, natural selection might be at work.
• It aids long-distance walking and running. Early hominins didn’t move bipedally as effectively or efficiently as we do, but bipedalism would have made the search for food or mates easier.
• It aids heat regulation. Bipedalism minimizes the amount of skin exposed to the sun and it could increase heat loss. However, the earliest hominins appear to have lived in forested or mixed forest-savanna settings, so this doesn’t explain why bipedalism arose.
• No single explanation is entirely satisfactory, probably because evolutionary processes have overlapping dimensions and complex effects on any species. What we do know is that as the end of the Pliocene and the beginning of the Pleistocene epoch approached, hominins became progressively better at walking on two legs.
• See “Anthropologist as Problem-Solver: Were We Born to Run?”
• Increased cranial capacity in hominins over several million years afforded greater brain power. Big brains require a lot of energy (upwards of 20% of the entire energy intake in modern humans), so the metabolic costs (in calories) for keeping the body operating also increased. If this began with H. habilis and H. rudolfensis, we would expect to see dietary changes toward more energy dense foods. All species of Homo, including us, are omnivores, but it is likely that expanding the amount of meat consumed in the diet could have helped meet the added energy expenditures of having larger brains. The consumption of meat is supported by findings in fossil sites.
• Paleoanthropologists believe that meat was a small portion of these early diets. The abundance and the relative ease of gathering roots, tubers, nuts, and fatty fruits as sources of high quality nutrition meant these were likely the staple elements of their diets.
• Increased brain capacity probably also supported cooperative social behaviors and group action, because it bolstered greater capacity for communication between individuals. Several fossil sites show that these groups consisted of several adults, children, and infants. They probably moved as groups and interacted with each other through grooming, gestures and vocal utterances.
• Paleoanthropologists hypothesize that among the early hominins bipedalism and increasing brain power, with associated changes in diet, tool use, and social relations, contributed to evolutionary changes that led to the later forms of Homo. If true, this hypothesis points to something quite powerful and new, the interaction of biology and culture through biocultural evolution to meet selective challenges.

Who Were The First Humans And Where Did They Live?

• The Pleistocene epoch reveals increasing qualities of “humanness” in our lineage, ranging from upright, large-brained physical qualities leading up to the modern body type we have today, with sophisticated language, complex social formations, and aesthetic imaginations. Our lineage also begins to spread out of Africa and populate other corners of the Earth.
• Originally, it was thought that humans came out of Asia, not Africa as is now known. The 1891 discovery of Pithecanthropus erectus, known popularly as “Java Man,” was later renamed and placed in the genus Homo became known as Homo erectus.
• H. erectus appeared about 1.8 mya, lived in numerous locations around the globe, and may have become extinct as recently as 30,000 years ago. They had human-like body proportions and height, lived their lives on the ground as obligatory bipeds, appear to have cared for their young and the weak, made and used stone tools, controlled fire, and may have even had some kind of simple proto-language.
• See “Classic Contributions: Davidson Black and the Brain Capacity of H. erectus”
• All of these qualities make H. erectus appear human. There were many obvious differences. Their bones were thicker and the skeleton more robust. The shape of the cranium was long and low, with a massive brow ridge. Some of the fossils also have a sagittal keel: a raised area in the mid-cranium.
• H. erectus fossils have been found throughout Africa, Europe, India, Indonesia, and China. It was long assumed that H. erectus evolved out of H. habilis, but recent finds in Kenya increasing H. habilis’ temporal range suggest that the two may have co-existed. The exact taxonomic ordering of H. erectus has never been totally resolved. There are three main hypotheses in the debate that continues today. The sparse nature of the fossil record and new findings and evidence challenge existing interpretations.
• Around 500,000-300,000 years ago, changes in both morphology and material culture suggest that one or more new variety of Homo had emerged. Classic H. erectus traits of robustness decreased and cranial capacity increased. Over the next several hundred thousand years, the brow ridges of these humans became smaller and more separated, with reduced postorbital constriction: an indentation of the sides of the cranium behind the eyes.
• These humans were also known for making tools that were more refined and specialized than previous tools, and individuals with these traits are referred to as archaic humans.
• Anthropologists classify archaic humans in one of two ways. The first is to lump them all into one category of archaic Homo sapiens, a broad category that assumes that we ourselves evolved directly out of this group. Another is to separate them into two different species, Homo heidelbergensis and Homo neanderthalensis. Regardless of their taxonomic status, archaic humans are almost—but still not quite—us.
• The oldest archaic human specimens are found in Africa. Their geographic spread includes the Middle East, Mediterranean, East Asia, Siberia, and Eastern and Western Europe.
• Much attention has been directed to fossils of Homo neanderthalensis found in many parts of Europe and the Middle East and dating from about 300,000 to 30,000 years ago. The discovery between 2008 and 2010 of a coeval archaic human dating to 41,000 years in Denisova cave in the Altai mountains of Siberia adds new evidence and interest in the ongoing debate.
• Neanderthals were stockier than modern humans, but in our same range of height and weight. Denisovans may have been similarly robust, but the sparseness of skeletal evidence (a finger bone and two teeth) prevents any morphological description and most of what we know about them comes from analyzing their mitochondrial DNA.
• There is strong fossil evidence that Neanderthals, Denisovans, and modern humans overlapped over a period of 10,000 years or more. Some sites in Europe and the Middle East have even yielded evidence of Neanderthals using modern human-like tool kits and modern humans using Neanderthal-like tool kits.
• Relationships among these varieties of Homo has grown more complicated and intriguing as ancient human genomics has developed technological innovations in sequencing, informatics, and improved recovery of mitochondrial DNA from fossilized bones less than 100,000 years old. There is sufficient DNA from the few Denisovan bones to reconstruct their genome. Ancient human genomics is still a new science and old DNA is often terribly degraded, but the latest findings point to some level of interbreeding and gene flow between Neanderthals, Denisovans, and modern humans, suggesting that the Neanderthals and Denisovans never completely disappeared because they shared genetic material with modern humans.
• Sometime between 200,000 and 25,000 years ago, the archaic features in the fossil record begin to change, including changes in morphology (a high, rounded cranium; relatively slender skeletal structure; and the appearance of a chin) as well as dramatic changes in the types and complexity of tools and other aspects of material culture and behavior. Language as we know it probably appeared with anatomically modern humans.
• 35,000-12,000 years ago, there may have been at least two species or subspecies of humans on the planet. Today all anthropologists agree that only one human species remains and all humans today belong to the same subspecies of Homo.
• Where we actually originated has led to the development of three explanatory models:
• The Recent African Origin model proposes that modern humans arose as a new species in Africa between 200,000 and 180,000 year ago, during the late Pleistocene.
• The Multiregional Evolution model proposes that modern humans are only the most recent version of a single species, Homo sapiens, that had been in Africa, Asia, and Europe for nearly 2 million years.
• The Multiple Dispersals model (MD) argues that modern humans left Africa in multiple waves, and edges out the others given the current fossil and DNA evidence. In this model the initial movement out of Africa occurs approximately 1.8 mya.
• In the MD model, Homo spread around western, central, and southern Eurasia, with back and forth gene flow across Africa and Eurasia, and possibly with some isolation of peripheral populations. Other dispersal patterns considered include:
• 800,000 to 400,000 years ago, influencing Eurasian populations through interbreeding, and affecting genetic patterns in the genus Homo.
• 150,000 to 80,000 years ago, also affecting genetic patterns throughout the genus Homo.
• 60,000 years ago, influencing the genetic patterns of central Eurasian and African populations. This event is also followed by back and forth gene flow as well as isolation by distance.
• 50,000 years ago, populations move into northern Eurasia, Australia, the Pacific Islands, and eventually the Americas (15,000 years ago) via migration.
• The MD model predicts that African populations would have had forceful and recurrent effects on the genetic characteristics of humans over the past 1.7 million years. It also predicts that gene flow would be an important recurrent dynamic, with groups living closer to each other most affected by that flow.
• It is difficult to pinpoint an exact dividing line between humans that are “anatomically modern” and archaic humans. Changes in complex organisms like humans may take hundreds or thousands of generations to occur. Closely aligned species can reproduce and pass genetic material between one another.
• Despite advances in DNA sequencing technologies, genetics and morphology alone will never give us a holistic picture of the first humans, because we also know that culture played a critical role in their evolution.

How Do We Know If The First Humans Were Cultural Beings, And What Role Did Culture Play In Their Evolution?

• The answer to this question is not that culture suddenly appeared and transformed everything. The cultural capacity of hominids emerged over a long period of time and interacted with biology to meet selective demands, in a process we call biocultural evolution: the interaction of cultural capacity and biology to meet selective demands.
• Humans began to tackle the challenges of the environment with something more than their hands and teeth during the Paleolithic: literally “old stone,” refers to a long epoch in human prehistory from
about 2.5 mya to 10,000 years ago, and roughly corresponds with the Pleistocene geological epoch.
• This resulted in real changes in the way the pressures of natural selection affected them. Human material and symbolic cultures continue to influence our evolution and reflect how much human biocultural evolution is a story of niche construction.
• Beginning with H. erectus, we see a greater capacity for culture. We know that culture played a greater role in their lives than earlier hominins because:
• Their diets changed. The increased brain and body size of H. erectus over previous Homo involved higher metabolic rates, requiring more and higher quality food. H. erectus responded to these new pressures by ratcheting up its reliance on tools and other cultural behavior to increase the quality of its diet. Support for this hypothesis comes from sites in East Africa where fossils of animals and stone tool assemblages appear around the time that early H. erectus fossils show up in the fossil record.
• Their tools changed. Early Homo first used Olduwan tools, which allowed them to process both animal and plant matter. About 1.6-1.4 mya, a new type of stone tool began to show up in the fossil record. Known as Acheulean tools (named after St. Acheul in France where they were first discovered in 1847), these tools had better edges and were more varied in style.
• They used fire. H. erectus and archaic humans created and maintained small fires. Evidence for fire exist in France, Spain, China, and Hungary, dating to 500,000 to 300,000 years ago. Fire enables the consumption of a wider variety of foods and a higher energy return on foods eaten. It also marks the beginning of cooking, a process that involves a transformation of raw food that anthropologists have long considered a unique hallmark of cultural capacity.
• Cooperative behaviors increased. H. erectus relied heavily on hunting and gathering high-quality plant material, both of which require coordination and cooperation among group members. With an increase in caloric requirements, the energetic costs of reproduction, and especially breast feeding (lactation), shoot up.
• None of these changes alone would have been sufficient to enable H. erectus to successfully adapt to a wide variety of landscapes across Africa and Eurasia, but together they generated an important adaptive capacity which played an even greater role in the adaptability and biology of archaic humans.
• Evidence for archaic human material culture and behavior comes from Neanderthal sites from between 300,000 and about 50,000 years ago. We see the emergence of more complex tools than the prior Acheulian style, and evidence of organized group hunts. Regular use of controlled fire was widespread, and we see evidence of shelters of wood and possibly hide.
• By 200,000 years ago, a new tool-making technique emerged. The Levallois technique involves flaking off pieces around the outline of a desired flake and produces higher-quality blades capable of many more uses. Many Neanderthal sites show an even more effective method for creating tools based on a disk-core technique called the Mousterian industry.
• All of these changes gave archaic humans improved access to nutrition, and new pressures on the body to process the richer foods. Cooking with fire took pressure off the need for big teeth and massive chewing muscles, and we see the gradual reduction of size in both of these in the morphology of modern humans.
• Everything we do as humans is based on social interdependence and intensive cooperation which in turn depends on communication well in excess of anything our chimpanzee cousins are capable of.
• By at least 300,000 years ago, members of the genus Homo were engaged in creative expression and symbolic thought.
• Having the capacity for culture was essential because cultural meanings allow for group memory, establishing patterned ways of doing things, and metaphysical thought. We cannot say with certainty when these characteristics appeared. It is impossible to really understand the non-material processes of culture based on material objects alone, so many crucial details about the culture of archaic and early anatomically modern humans remain unknown.
• We know that archaic humans such as Neanderthals exhibited social interdependence and appeared to have symbolic lives rooted in come level of linguistic ability. They had jewelry, possibly cared for the sick and injured, and treated their dead with reverence.
• By at least 50,000 years ago, anatomically modern humans were creating images on cave shelter walls and rocky outcrops that some scholars interpret as art. Over the next 20,000 years, art and other symbolic forms became much more common. The type of imagery in early visual art includes both abstract and realistic representations. While it is difficult to know why these things were created and if they had some intentional purpose, they indicate that by 50,000-30,000 years ago, all humans were fully capable of symbolic thought, and almost certainly language.
• Contemporary humans’ cultural and symbolic capacities developed over a very long time and it is clear that cultural adaptations also shaped the evolutionary possibilities and characteristics of archaic humans and anatomically modern humans.

Conclusion

• Increasing amounts of ancient primate fossils as well as technological advancements in the lab allow researchers to access the subtle information about human origins. The rarity of primate and hominin fossils leaves us much to learn about the lives of those ancient creatures and their relationships to us.
• While we cannot yet say for certain that any specific fossil older than 2 million years represents our direct ancestral species, we do know that the human lineage arose in Africa and emerged to confront the ecological challenges of the planet in a totally new way.
• While fossils, genetics, and morphology are critical elements of biological anthropology, alone they will never give us the holistic picture of humans that anthropologists strive to develop, because we also know that culture played a critical role in the evolution of the first humans.