Recall that the ends of the ‘branches’ on the evolutionary/taxonomic tree are Order → Family → Genus → Species. In the previous chapter, we looked at the order of primates. In this chapter, we will look at the hominins (a “tribe” between family and genus) with an eye toward the genus Homo which is today home to a single species—our own, Homo sapiens. This genus once included multiple species we might consider ‘biological humans.’ Then again, if we thought the order of primates complicated what it means to be ‘human,’ trying to draw lines within our genus (or even tribe) is even trickier!
One thing to keep in mind as you read this chapter: we have a tendency for linear thinking. Today, scientists understand that human evolution was truly not a linear process. That said, this chapter still largely presents it like that for two reasons. First, especially at the introductory level, it is easier to wrap our heads around. Second, while we now know that the “braided stream” analogy fits better than the “evolutionary tree” analogy, we do not yet know enough about how earlier species interacted. It’s a tradeoff. We know that there was a braided stream, but to guess how it was braided would paint a picture less accurate than the more linear version.
On the subject of time, it might be helpful here to present or review some terms which will be used in the remainder of this chapter. These terms are often shorthand for paleoanthropologists interested in early human ancestors and ‘modern’ biological humans.
Miocene: A geologic epoch. 23 to 5.3 million years ago. Often divided into three parts: early (23-16 mya), middle (16-12 mya), and late (12-5.3 mya).
Pliocene: A geologic epoch following the Miocene. 5.3 to 2.6 million years ago. Often divided into two parts: early (or Zanclean, 5.3-3.6 mya) and late (or Piacenzian, 3.6-2.6 mya)
Pleistocene: A geologic epoch following the Pliocene. “The Ice Age.” 2.6 mya to 12,000 years ago. Divided into three parts: early (2.6 mya-774,000 years ago), middle (774,000-129,000 years ago), and late (129,000-12,000 years ago).
Paleolithic: A paleontological and archeological era lasting from 3.3 mya to 12,000 years ago. This era covers much of the late Pliocene and all of the Pleistocene. This is also known as the “Stone Age” (the word “paleolithic” comes from the Greek for ‘old stone’). Divided into three periods: Lower (3.3 mya-300,000 years ago), Middle (300,000-50,000 years ago), and Upper (50,000-12,000 years ago).
Neolithic: An archeological era lasting from 12,000 to 3,000 years ago. The agricultural age for many humans. The name means ‘new stone.’
Australopithecus & Paranthropus
We use term ‘hominin clade’ to refer to all species considered to be in direct lineage to humans, which include the genera Homo, Australopithecus, Paranthropus, and Ardipithecus. Do not be confused by the shorthand ‘hominin’ to refer to this clade given that the ‘hominines’ today include humans and chimpanzees + bonobos. These terms have been understood to represent different things over the years, but in this chapter, hominids refers to all modern and extinct great apes, which include humans, gorillas, chimpanzees, and orangutans and their ancestors. Homininsrefers to the hominin clade—the human-est species there have been.
The first hominid fossils appear in the late Miocene, 10 to 5 mya. Sometime between 7 mya and 4 mya, hominids moved out of the trees and began to adapt more fully to a ground-based living niche. Sahelanthropus tchadensis lived approximately 7 mya in west Africa and is thought by some to be the last common ancestor of humans and chimpanzees. Genetic studies indicate that this species lived at the time of the divergence, but morphology suggests that Sahelanthropus was not bipedal. Orrorin tugenensis dated to approximately 6 mya was found in Kenya and was proposed to be a hominin due to anatomical traits that suggest bipedalism. Another feature that suggests that Orrorin was truly a hominin is the teeth, which exhibit thick dental enamel and small, square molars, much like modern humans.
While all hominins may differ in varying ways from one another, they all share one anatomical behavioral complex: bipedal locomotion. Scientists can hypothesize about how a creature moved by analyzing several aspects of its morphology. A variety of indicators show bipedalism including: the lengths of the legs in relation to the arms, the arch and relative length of the foot, big toes parallel to the others, the angle at which the femur (upper leg bone) meets the pelvis (hip bones) and at which it angles straight down or inward, and spinal curves that make it possible for the hips to balance the weight of the upper body.
The evolution of hominin bipedalism required complex anatomical reorganization. For natural selection to produce such a tremendous amount of change, the benefits of these changes must have been great. There have been dozens of hypotheses for these changes, ranging from freeing hands to carry tools, food, or offspring to increasing energy efficiency or the ability to maintain the body’s temperature (thermoregulation) by exposing more of the body’s surface.
The Pliocene epoch extended from 5 mya to 1.8 mya. Fossils from the Pliocene show evidence of clearly bipedal hominins. They also show evidence of cultural behavior. Ardipithecus ramidus (‘Ardipithecus’ means ‘earth/ground ape’ and ‘ramidus’ means ‘root’) was found in Ethiopia and dated to about 4.4 mya. Their morphology suggests bipedalism. Ardipithecus possesses numerous traits, such as thin dental enamel, evidence of a reduced canine, and an opposable big toe.
The next species to look at are ‘australopithecine’ species. Australopithecus means “southern ape.” This group had diverse physical characteristics related to morphology of the teeth and skull. Based on these characteristics, paleoanthropologists classified these species into gracile and robust forms classified under Australopithecus (Australopithecus anamensis, A. afarensis, A. africanus, A. garhi, and A. sediba) and Paranthropus (Paranthropus robustus, P. boisei, and P. aethiopicus) respectively. The gracile species emerged around 4 mya and disappeared 2 mya, while robust species continued to exist for another million years.
The fossil known as Lucy. Australopithecus afarensis. [Source: "120", Wikimedia Commons]
In 1973, about 40% of a skeleton was found in Ethiopia by paleoanthropologist Donald Johanson. He famously called the skeleton Lucy, after a Beatles song. It was dated to around 3.75–2.8 mya and was determined to be a member of the species Australopithecus afarensis. Since then, more specimens of this species have been found in Kenya, Tanzania, and Ethiopia, all in East Africa dated from 3.9 mya. Morphological features like longer arms and feet suited for branches suggest A. afarensis moved more like other great apes than humans. However, the shape of their pelvis (hip bones) looks substantially more like a modern human’s than it does an ape’s. Most paleoanthropologists believe that this change in pelvic shape indicates they moved on two legs if not in exactly the same way humans do. Current consensus is that A. afarensis was both arboreal and bipedal.
The Taung skull/child. Australopithecus africanus. [Source: Emőke Dénes, Wikimedia Commons]
A notable specimen of Australopithecus africanus(3.3 to 2.1 million years ago) is now known as the Taung skull or Taung child. With a brain like a chimpanzee but other human-like features it was once assumed to be an intermediary step on the way to becoming human. Most importantly, scientists at the time of the Taung skull’s discovery noted that the forward position of the foramen magnum indicated that the skull was poised on top of the vertebral column, suggesting bipedalism and an upright posture.
Reconscructed cranium of Australopithecus garhi. [Source: 宜蘭第一公民, Wikimedia Commons]
Also found in Ethiopia, Australopithecus garhiis dated to approximately 2.5 mya. Its brain was slightly larger than A. afarensis. A garhi’s incisors are larger than those of any of the known australopithecines or Homo. The function of the large incisors is not yet known. The most exciting aspect of A. garhi is that it provides evidence of the earliest use of stone tools by a hominin as fossils were found those of ruminants, such as antelopes displaying numerous cut marks consistent with butchering.
Australopithecus sediba was discovered in a cave in South Africa and is considered an important species because it appears in the fossil record around 2 mya—around the time the genus Homo’s emergence. The classification of A. sediba was initially difficult to determine, due to its complex overlapping features, which include humanlike spine, pelvis, hands, and teeth but a chimpanzee-like foot. Classified as a species of Australopithecus, A. sediba is considered a direct ancestor of Homo erectus and Homo ergaster. A. sediba could be a descendent of A. africanus, which suggests the species may be a dead end within the lineage to humans.
Two species of Paranthropus (more robust cousins to Australopithecus) includeP. aethiopicus (2.7 to 2.3 mya)discovered in Ethiopia and P. boisei (2.5 to 1.1 mya) discovered in the Eastern Rift Valley (a 1,200-mile continental rift zone through Ethiopia, Kenya, and Tanzania). P. aethiopicus is thought to be somewhere between the ‘robust’ and ‘gracile’ australopithecines while P. boisei is sometimes described as ‘hyper-robust.’
Paranthropus robustus (from 2 to 1 mya) was also discovered in South Africa. The most obvious difference between A. africanus and P. robustus is that robustus is more—well—robust! Its features include large skull attachments for chewing muscles indicating a diet reliant on hard nuts and seeds. This interpretation was further supported by scanning electron microscopy (SEM) used to evaluate the markings etched into the teeth. As the teeth increased in size the incisors and canines shrank, giving Paranthropus a flatter face with less projection of the jaw. There are some who argue that depending on the environment and locale, some Paranthropus may have been omnivores, with varied diets similar to those of H. ergaster.
Genus Homo
We are increasingly discovering that the evolution of the genus Homo is more complex than previously thought. The earlier view of a simple progression from Australopithecus (a name meaning ‘southern ape’) to Homo habilis (‘handy man’) to Homo erectus (‘upright man’) to Homo sapiens (‘wise man’) as clearly delineated stages no longer holds up.
Rather, scientists have largely concluded there was tremendous variability during the last four million years. While fossils classified as H. habilis, for example, show many of the characteristics of the genus Homo, such as brain expansion and smaller tooth size, their small body size and long arms are more akin to australopithecines. There is also tremendous variability within the fossils assigned to H. habilis, so there is little consensus on whether it is one or multiple species of Homo, a member of the genus Australopithecus, or even a yet-to-be-defined new genus. Similarly, there are considerable differences in skull morphology and body size and form of H. erectus, of which some specimens show more similarity to H. habilis than previously thought.
What does this diversity mean for how we should view early Homo? Which are our ancestors? Which are our close cousins? How often were they both? First, there isn’t an abrupt break between Australopithecus and H. habilis or even between H. habilis and H. erectus. Characteristics we define as Homo don’t appear as a unified package; they appear in the fossil record at different times. This is known as mosaic evolution. Indeed, fossil species such as A. sediba, as well as H. naledi and H. floresiensis, have displayed unexpected combinations of early and later ‘human’ traits.
There is no universal consensus on what is included within the term “archaic Homo.” The term is used as an umbrella category encompassing all the diverse Homo species after H. erectus. H. erectus lived during the Pleistocene (spanning 2 mya to 120,000 years ago) and was the first human species to evolve a humanlike body plan and gait, to leave Africa and colonize Asia and Europe. Archaic Homo are often distinguished from anatomically modern humans by the characteristics of a thick skull, prominent brow ridges, and lack of a prominent chin. Archaic Homo are viewed as transitional between H. erectus and H. sapiens and display many overlapping and varied traits. It has been proposed that archaic Homo may have been the first species to use language, based on the size of their brains and the fairly large social groups they lived in.
A key characteristic shared by both the australopithecines and the Homo is bipedalism. But what criteria other than bipedalism might be used to classify a species under the genus Homo? Many anthropologists have attempted to establish specific criteria to use in determining a classification of Homo.
As with any definition, this is further complicated by a sort of chicken-and-the-egg effect. We might say Homo is defined by bipedalism, tool use, large family/political structures, features of the skull, or certain brain structures, but are these common to a subset of species we happen to call Homo or is the other way around? Put another way, if we found a species with all these features but lacking features of the skull, would we call them Homo or redefine the genus?
Skull of H. habilis. In contrast to Paranthropus or Australopithecus, habilis displays increasingly "Homo" gracility. [Source: Gunnar Creutz, Wikimedia Commons]
One strategy paleontologist used is to rely on a ‘type specimen’ as reference. For example, Homo habilis (dated between 2 and 1.7 mya) is considered to be one of the earliest species in the genus Homo. The type specimen H. habilis was found in 1960 at Olduvai Gorge by Jonathan and Mary Leakey. The specimen consisted of a partial juvenile skull, hand, and foot bones. It possessed teeth that were much smaller than those of any australopithecine and was possibly in coexistence with Paranthropus. Based on an imprint of the interior of the brain case (endocranial cast), it was determined that H. habilis may have possessed what is called a Broca’s area in the brain. The Broca’s area is especially important for speech development leading some scientists to suggested H. habilis started to develop the neural networks necessary for human speech or possibly already had speech itself.
The Broca's area's position indicated in red. [Source: Database Center for Life Science, Wikimedia Commons]
By looking at the inside of the skull, paleontologists can learn a lot about the brain that used to reside there. In paleoanthropology, encephalization refers to a progressive increase in brain size over time and which is assumed to correlate with an increase in behavioral, cognitive, and cultural complexity. Cognitive developments correspond with our ability to construct and form ideas, including the ability think and communicate with symbolic and abstract language, such as that used in storytelling, ritual, and art. There are always exceptions, however, such as the island-dwelling, small-brained H. floresiensis. Despite having a very small brain, H. floresiensis made and used tools and built fires. This discovery has challenged what we thought we knew about the correlation of brain size and cognitive development in human evolution. As we learned about primates in general in the last chapter, however, brain functions may have more to do with the complexity of the brain than its overall size.
The encephalization quotient (EQ) (or size of brain in relation to expected brain given body size) can serve as at least a baseline indicator of classification within the genus Homo. In short, the larger the brain weight relative to the overall body weight, the more likely that the brain was used for more complex cognitive tasks. While EQ is a strong tool for studying brain size in early hominins, there are always potential margins of error when dealing with fragmentary fossils, and increasingly scientists propose alternative forms of measurements. Some hypotheses consider the number of cortical neurons and neural connections the most important when considering cognitive ability—superseding size or EQ so much as to make them unreliable. According to this approach, the density of the cortex is more associated with intelligence than is brain size. And these alternate approaches perhaps better explain those exceptions in the fossil records.
This chart maps the trend in brain size among hominids/Homo over the last 10 million years. Note the call-out box showing the explosion of fossil evidence over the last 100,000 years and clustering of brain size therein. Outliers referenced in this chapter (H. naledi and H. floresiensis) are also indicated. [Source: DeSilva, et al., frontiersin.org via Wikimedia Commons]
One critique of paleoanthropology is the tendency to privilege or over-rely on observations about the brain. Unfortunately, portions of skulls are more common in the fossil record than the rest of skeletons. The features of the rest of the body are referred to as postcranial features. In some cases, portions of a skull are all we have to go on. In other cases, all we have are postcranial features (a clavicle and a few fingers, for example). This can be equally frustrating for paleoanthropologists because our definitions of hominin species often rely on a combination of both types of data. The best ‘type specimens’ therefore have some of both. Many ongoing debates among paleoanthropologists about hominin species center around how to best classify species based on the limited data.
For example, H. habilis has been at the center of several debates regarding their taxonomic position and relationship with other early archaic Homo species. For example, H. habilis was initially believed to have been a direct human ancestor through the lineage of Homo erectus and then modern humans. This viewpoint is now debated and has resulted in a scientific divide between those supporting H. habilis and those suggesting another Homo species, H. rudolfensis, as being the ancestor of H. erectus. H. rudolfensis is an archaic Homo dated to about 2 mya, which coexisted with other Homo species during that time period. A cranium was discovered in in Kenya, but scientists are missing postcranial materials. There are hypotheses that propose that H. rudolfensis might be a H. habilis male, exhibiting a larger cranium than that seen in a female H. habilis. Others suggest it is a completely different species.
While there are still questions as to the phylogenetic relationship of H. habilis and H. rudolfensis, there is general agreement that Homo did evolve from Australopithecus. The timing and placement of the split between Australopithecus and Homo, however, is still debated.
All this is to say, attempts at defining Homo or any of the species within it is complex, let alone trying to determine the particular lineages between them (with or without australopithecines in the mix). For our purposes, we will have to be content taking several species today grouped among Homo as examples and pieces of a complex puzzle recognizing that mosaic evolution, the braided stream, and even our own definitions of Homo will likely alter our understanding of how these pieces fit together.
Ancient Diversity
Comparison of skulls over the last 3.3 million years from A. africanus to H. sapiens and "anatomically modern humans." [Source: SimplisticReps, Wikimedia Commons]
Homo antecessor (1.2 to 0.8 mya) has been found in Spain, France, and England. They represent the oldest fossil evidence for the presence of the genus Homo in Europe. H. antecessor was found in Spain within a cave known as “the Pit of Bones,” where fossils of 28 individuals have been found that date at or before 780,000 years ago. The site is an important one that stretches over a long period of time and displays the emergence and divergence of various Homo physical characteristics that later appear in the Neanderthal. Morphologically, it might even have been possible to mistake H. antecessor for modern humans or Neanderthals. One very modern trait exhibited by this species is the presence of a facial depression above the canine tooth called the canine fossa, which is also found in modern humans.
H. antecessor has also been found with extensive stone tools and animal bones Cutmarks are present on the majority of animal remains that are believed to have been transported to the site rather than consumed where they were killed. Some suggest the practice of bringing food back to the site is evidence of social cooperation, suggesting both a division of labor and a custom of food sharing. Oddly, many of the bones of Homo antecessor show the same evidence of cutmarks as the animal bones, indicating that flesh was removed from the bones with the goal of dismemberment. Some scientists have taken this to mean that H. antecessor practiced cannibalism. However, humans have also been known to remove the flesh from bones during funerary rites. Whether the cutmarks made by H. antecessor represent cannibalism, a funerary rite, or another yet unknown practice is still being debated.
Homo naledi (235,000–335,000 years ago) were found in the Rising Star cave system in South Africa in the 2010s. Bones from as many as 15 individuals were recovered from the cave. Despite their relatively recent date, they have exceptionally small cranial capacities, comparable to australopithecines, notably smaller than all other Homo. The presence of this small-brained hominin at the same time that Neanderthals and H. heidelbergensis were around is further evidence that multiple hominin lineages were coexisting and evolving at the same time. The classification of H. naledi has proved to be a challenge. They show no evidence of stone tool use, but there is evidence (equally convincing and controversial) that H. naledi used the cave system as a place to bury their dead.
The Denisovans (named for where they were discovered, Denisova Cave in Siberia, Russia) are archaic Homo thought to have lived between 500,000 and 30,000 years ago. Because of this lack of fossil evidence, very little is known of their anatomical features. A jaw and two teeth found in a cave on the Tibetan Plateau (in China) was dated to 160,000 years ago. Protein analysis determined the jaw to be Denisovan and from a member of a population who were most likely well adapted to living in high altitudes.
Because so few bones have been found, most understanding of this species comes from genetic analyses. According to nuclear DNA studies, Denisovans and Neanderthals were more closely related to each other than they were to modern humans. DNA evidence suggests that the Denisovans interbred with modern humans and with local Neanderthal populations over multiple time periods. H. heidelbergensis is typically considered to have been the direct ancestor of both Denisovans and Neanderthals, and sometimes also of modern humans.
One specimen called “Denny” is a first-generation hybrid with a Denisovan father and a Neanderthal mother. Denny indicates Late Pleistocene Homo species interbred when the groups met. Comparison of the DNA of these three groups suggest that most modern-day Europeans and Asians inherited about 1–4 percent of their DNA from Neanderthals, with no Denisovan ancestry in Europe and 0.1 percent in China. The genetics found in Tibetans, Melanesians, and Indigenous Australians are currently being challenged; originally, they were thought to be about 3–5 percent Denisovan and 2.74 percent Neanderthal, but new evidence suggests there could be a third group yet unknown contributing to the Pacific Islander genome. Statistical and genetic evidence can serve as indicators of the existence of this kind of group for which no fossils have yet been found. These are referred to as ghost populations. For example, there are indications that 2–19 percent of the DNA of four West African populations may have come from an unknown archaic hominin that split from the ancestor of humans and Neanderthals between 1 mya and 360,000 year ago.
Recently a new archaic Homo fossil surfaced in Harbin, China, dated to about 146,000 years ago. H. longi, “Dragon man,” or “the Harbin cranium” may represent a lineage of the Denisovans or a new species, but it is clear it was robust and able to adapt to one of the coldest regions of China. It had a large brain, thick brow ridges, and fairly large teeth, similar to what is found in the Denisovans.
Homo heidelbergensis is an incredibly variable group. Many archaic Homo species are included in this group because they possess features that can best be described as a mosaic between H. ergaster, H. erectus, and “anatomically modern humans.”
One H. heidelbergensis individual, known as Mauer, was found in Germany and is represented by a mandible (lower jaw) that is dated to approximately 600,000 years ago. It has a robust mandible and a receding chin like earlier Homo ergaster but has very small molars like anatomically modern H. sapiens. The jaw is so big and the teeth are so small that there is plenty of space for additional teeth to develop behind the wisdom teeth. Given that the third molar (the wisdom tooth) has already erupted, it has been suggested that this individual was between 20 and 30 years at death. Another, known as the Petralona cranium, was found in Greece with ambiguous dates between 700,000 to 100,000 years ago but animal fossils found with Petralona indicate Petralona is between 350,000 and 200,000 years old. Petralona combines H. ergaster-like traits with a cranial capacity like anatomically modern H. sapiens. A third, known as Bodo, found in Ethiopia, is very possibly the oldest archaic human specimen from Africa and is dated to approximately 600,000 years ago. Bodo’s cranial capacity is within the range of variation for modern humans. It is a robust cranium with very thick bones and two separate brow ridges.
The mandible of the H. heidelbergensis individual known as Mauer. [Source: Gerbil, Wikimedia Commons]
Bodo was found with hand axes suggesting the butchering of animals. There are cutmarks on the Bodo cranium that resemble those made by cutting fresh bone with stone tools. It has been suggested that the Bodo cranium is the earliest evidence of the removal of flesh immediately after death using a stone tool. The cutmarks were made symmetrically and with specific patterns on the cranium, which is interpreted as strong evidence that the defleshing was done purposefully for funerary practices. Once again, others have suggested that the cutmarks indicate that Bodo may have been practicing cannibalism.
In addition to their use of stone tools, H. heidelbergensis is also believed to have used spears about 400,000 years ago. The spears were made using ‘hafting’ technology which involves attaching stone points to a handle made of another substance, such as wood Hafting gives stone tools more utility, as they can now be thrown (as with a spear), shot (as with an arrow), or used with more leverage (like an axe). This increased efficiency in hunting and killing animals is believed to have created a situation in which H. heidelbergensis had regular access to meat and other high-quality foods. Another interesting aspect of H. heidelbergensis is that they are associated with clear archeological evidence for modified dwellings consisting of stone foundations as well as controlled fire dated to around 780,000 years old.
Homo floresiensis, also known as “the Hobbit (of Flores)” was discovered on the island of Flores in Indonesia in 2003. The species has been dated to approximately 100,000–60,000 years ago. What was surprising about this species is its small size—adults under 4 feet tall. The cave where H. floresiensis was found, shows evidence of the use of fire for cooking and contains bones with cutmarks. As a result of anatomical differences, it is believed that their bipedalism was quite different from that of modern humans, with a high stepping gait and slower walking speed. H. floresiensis also had substantially more mobility in the elbow joint, which suggests that they were good tree climbers. Their small brain size is not believed to have affected their intelligence. The brain of H. floresiensis does contain a Brodmann area (associated with cognitive abilities) that is the same size as that found in modern humans. Some have suggested that H. floresiensis is a sister species of Homo habilis that branched off before or shortly after the evolution of Homo habilis. Other hypotheses suggest that they were the descendants of H. erectus who became stranded on the island.
Homo luzonensis, found on the island of Luzon in the Philippines and dated to at least 50,000–67,000 years ago displays a hybrid of australopithecine traits and Homo characteristics, yet lived alongside modern H. sapiens. Clearly the genus Homo is more diverse and complex than was originally thought, especially within the special evolutionary pressures of island environments.
Map of estimated periods of Homo migration over the last 170,000 years. Note that this map 'peels' the globe from the south pole and looks down from the north pole. Africa is in the top left, upper Asia and upper North America are in the center, South America is to the right. The colored legend indicates thousands of years. Importantly, this map shows the multiple 'phases' of migration into Asia as well as the America during which times the 'braided stream' occurred. Missing from this illustration are now-hypothesized return(s) to Africa. [Source: Wikimedia Commons]
“Neanderthals” have been found only in regions of Europe and the Middle East and are dated to between about 400,000 and 40,000 years ago. The first fossils, which were found in the Neander Valley, were believed to be the remains of an extinct kind of human. They possess several distinctive anatomical characteristics: the skull and brain are larger than that of humans. It is believed that in the Neanderthal brain, the frontal region, which is the center of speech and language, was less developed, while the back of the brain, which deals with the senses, was more developed. This greater development in the back area of the brain could be a survival adaptation found in Neanderthals who had to hunt in often harsh and difficult conditions. Given their morphology, the verdict is still out as to whether they were able to produce complex language similar to H. sapiens. It is believed by some researchers that the ability to produce complex speech gave H. sapiens their significant edge over the Neanderthal.
Some of the best-known Neanderthal specimens come from a place called Shanidar Cave in Iraq. Within this cave, various skeletal remains of ten individual Neanderthals were found. Nearly all the skeletal remains show some evidence of trauma, suggesting that hunting was risky business. In a comparative study, it was established that during the Upper Paleolithic, modern H. sapiens sustained similar injuries as the Neanderthal, but interestingly, these injuries were less likely to result in death. Shanidar also features individuals who suffered grave injury but also suggest healing by care from others. Some paleoanthropologists also conclude there is evidence of intentional burials (deliberate placing of the dead in a ritualistic manner). It is hotly debated, but Pollen analysis of the soil surrounding one corpse suggests that spring flowers had been placed in the grave, possibly indicating that the Neanderthal had a belief in an afterlife and established mortuary practices. These two features suggesting compassion and a sense of social responsibility for at least the disabled and quite possibly the deceased members of the community.
Neanderthals have been labeled, perhaps unjustly, as a species with a limited ability to communicate in symbolic or abstract forms. Until recently, the Neanderthal had been assumed to lack the cognitive skills associated with the practice of ritual and art. However, cave paintings discovered in Spain challenge that assumption. These paintings, which have been dated to around 65,000 years ago, before the arrival of H. sapiens in the region, have been determined to be the creative works of the Neanderthal and are currently considered the oldest cave art ever found. Neanderthals created more technologically advanced tools than those produced by H. erectus. Archeological sites that date to the Neanderthal period are dominated by flake tools. This means that the Neanderthal struck flakes from cores and then used the flakes as their tools instead of the core. This resulted in smaller and sharper tools with increased utility—but ones that are more difficult for us to find!
Known distribution and significant discovery locations of Neanderthals through portions of modern-day Asia and Europe. Shanidar is roughly central to the shaded area. Today, the known range extends even further north than indicated here. [Source: Berria, Wikimedia Commons]
The Neanderthal went extinct around 35,000 to 50,000 years ago. There have been various hypotheses as to what caused this, many connected to the fact that Neanderthal coexisted with H. sapiens in regions of Europe and Asia for an estimated 2,600–5,400 years. These hypotheses include an inability to adapt to a changing climate and colder temperatures, the spread of disease, competition for food with H. sapiens, and even aggressive takeover by the H. sapiens, who may have been better able to adapt to environmental changes. Another theory points to evidence that the Neanderthal tended to live in small, scattered groups with limited genetic diversity and low birth rates, which potentially impacted the ability of the Neanderthal to be competitive.
And then some argue that the Neanderthal didn’t go extinct at all because some people still have Neanderthal genes in them. Recent genetic evidence indicates that human-Neanderthal interbreeding was happening as far back as 125,000 years ago. Various mutations and diseases are linked to this Neanderthal DNA, including diabetes, addictions, depression, allergies, and Crohn’s disease. Neanderthal genes are believed to have provided immunity to some viruses that H. sapiens, arriving from Africa, would not have had time to build up an immunity against. On the reverse side, H. sapiens may have brought diseases from Africa that the Neanderthal did not have resistance to, possibly playing a role in their extinction.
The Braided Stream and Assimilation
Comparison of modern H. sapiens (left) and H. neanderthalensis (right) skulls. [Source: hairymuseummatt (original photo), DrMikeBaxter (derivative work), Wikimedia Commons]
We can consider several explanations for the diversity we see within early Homo from about 2.5 million to 1.5 million years ago. One possibility is the existence of multiple contemporaneous species of early Homo during this period. In light of environmental instability, it shouldn’t be surprising to see fossils from different parts of Africa and Eurasia display tremendous variability. Multiple hominin forms could also evolve in the same region, as they diversified in order to occupy different ecological niches. However, even the presence of multiple species of hominin does not preclude their interacting and interbreeding with one another.
Various lines of evidence about the genus Homo point out the need for a more intricate view of this period of human evolution. Rather than obvious demarcations between species and their corresponding behavioral advancements, it now looks like many behaviors were shared among species. Earlier hominins that we previously didn’t think had the capability could have been doing things like expanding out of Africa or using stone tools. Meanwhile, some other hominins that we had considered more advanced didn’t actually have the full suite of “human” characteristics previously expected.
To further complicate things, recently hypothesized interbreeding between nearer species produces a version of hominin evolution which looks more like a ‘braided stream’ than a branching tree. The braided stream model of modern human evolution proposes a more complex, interconnected network of diverging and re-converging lineages, rather than a simple linear tree. Fossil and DNA evidence show that hominin populations coexisted and interbred (hybridized) to exchange genes both Africa and as they moved beyond it. The genetic contributions of “early” humans are not straightforward. This model better reflects the ebb and flow of genetic exchange in the hominin gene pool.
A literal 'braided river.' While portions of the river split and re-join the overal path of the river from source to end is still one river. [Source: Laurent Deschodt, Wikimedia Commons]
Despite some haziness dominating the early Homo narrative, we can identify some overall trends for the million-year period associated with early Homo. These trends include brain expansion, a reduction in jaw protrusion (called facial prognathism), smaller jaw and tooth size, larger body size, and evidence of full terrestrial bipedalism. These traits are associated with a key behavioral shift that emphasizes culture as a flexible strategy to adapt to unpredictable environmental circumstances. Included in this repertoire are the creation and use of stone tools to process meat obtained by scavenging and later hunting, controlled fire and cooking, further prolonged childhoods, cooperation in child raising, and the practice of skilled foraging techniques. In fact, it seems cultural innovations drove biological changes, and vice versa, fueling a feedback loop that continued during the later stages of human evolution.
How do researchers make sense of all of these discoveries that cover over at least a million years of time and stretch across every continent except Antarctica? How was modern Homo sapiens related to archaic H. sapiens? The Assimilation hypothesis proposes that modern H. sapiens evolved in Africa first and expanded out but also interbred with the archaic H. sapiens they encountered outside Africa. This hypothesis is powerful since it explains why Africa has the oldest modern human fossils, why early modern humans found in Europe and Asia bear a resemblance to the regional archaic humans, and why traces of archaic DNA can be found in our genomes today. But while researchers have produced a model that satisfies the data, there are still a lot of questions for paleoanthropologists to answer regarding our origins. What were the patterns of migration in each part of the world? Why did the archaic humans go extinct? In what ways did archaic and modern humans interact? The definitive explanation of how our species started and what our ancestors did is still out there to be found.
“Modern” Humans
Modern Homo sapiens first appeared about 200,000 years ago in Africa. Anthropologists generally classify these people as “anatomically modern H. sapiens,” which is a way of noting that while their bodies are mostly the same as modern humans, they had not yet developed the cultural traditions, symbolic behaviors, and technologies that are seen among later H. sapiens. Probably the most defining feature of anatomically modern H. sapiens is their chin. Modern H. sapiens is the first hominin to exhibit a projecting chin. One of the most common explanations for this anatomical feature is that the chin evolved in response to human speech and protects the jaw against stresses produced by the contraction of certain tongue muscles.
Sometime around 40,000 years ago there was an abrupt change in tool technology, subsistence patterns, and symbolic expression among H. sapiens. These changes seem to have occurred almost simultaneously in Africa, Asia, Europe, and Australia. While there is evidence of some creative artistic activity in earlier groups like the Neanderthal, they were not on the same scale as that seen during the Upper Paleolithic. Among these changes, H. sapiens began assembling much more elaborate tools. Over the 23,000 years of the Upper Paleolithic, there were many distinctive periods of tool use (archaeologists call these periods “industries”).
The Gravettian tool industry showed many instances of animal remains being used for both decorative and traditional tool purposes. The Gravettian tool industry is best known for carved Venus figurines portraying a woman, typically made from ivory or limestone. Most figurines have small heads, wide hips, and large breasts. Most researchers believe that they served a ritual or symbolic function. Some have suggested that they represent an expression of health and fertility.
The Solutrean tool industry utilized tool-making techniques not seen before. It produced finely worked points with new techniques. The Solutrean is also characterized by the appearance of the atlatl—a long stick used to propel a spear or dart. Functioning as an extension of the arm, this stick of wood or antler added range to a short spear. By 17,000 years ago, the Solutrean tool industry was replaced by the Magdalenian. During this period, bone and ivory continue to be used, as well as stone.
During the Upper Paleolithic, H. sapiens created a great deal of cave art. Cave art seems to have been created continually from 40,000 to 10,000 years ago and then disappeared around 10,000 years ago, likely due to climate change. The most well-known cave sites in France are the Chauvet (32,400 years ago) and Lascaux Caves (17,000 years ago). The art in both caves features common subjects, such as bison, horses, and deer, as well as tracings of human hands. Most of the animals depicted were commonly hunted but were not always found with associated deposits of bones. The cave art produced during the Upper Paleolithic show a level of sophistication, sacredness, and possibly even a form of astronomy not seen previously in human history.
A bull is one of the many animals depicted in Lascaux cave. Interestingly, the dots indicated above this one's shoulders may well correspond to a cluster of stars known as the Pleiades—located in exactly the same relationship to the modern constellation Taurus (the bull). One modern theory of Lascaux is that these paintings served as a sort of 'calendar' for hunting. [Source: JoJan, Wikimedia Commons]
What defines modern H. sapiens when compared to archaic H. sapiens? Modern humans, like you and me, have a set of derived traits that are not seen in archaic humans or any other hominin. As with other transitions in hominin evolution, such as increasing brain size and bipedal ability, modern traits do not appear fully formed or all at once. In other words, the first modern H. sapiens was not just born one day from archaic parents. The traits common to modern H. sapiens appeared in a mosaic manner: gradually and out of sync with one another. There are two areas to consider when tracking the complex evolution of modern human traits. One is the physical change and the other is behavior inferred from these physical traits and material culture among them.
The skeleton of what are called “physically modern Homo sapiens” has structures that are thinner and smoother (called ‘gracile’). Modern skulls are much rounder, or more globular. You can feel the globularity of your own modern human skull. Viewed from the side, the tall vertical forehead of a modern Homo sapiens stands out when compared to the sloping archaic version. This is because the frontal lobe of the modern human brain is larger than the one in archaic humans, and the skull has to accommodate the expansion. The trend of shrinking face size across hominins reaches its extreme with our species as well. Continuing a trend in hominin evolution, technological innovations kept reducing the importance of teeth in reproductive success. As natural selection favored smaller and smaller teeth, the surrounding bone holding these teeth also shrank. Related to smaller teeth, the mandible is also gracile in modern humans when compared to archaic humans and other hominins. Interestingly, our mandibles have pulled back so far from earlier hominins that we gained an extra structure at the most anterior point, called the mental eminence. You know this structure as the chin.
The rest of the modern human skeleton is also more gracile than its archaic counterpart. The differences are clear when comparing a modern H. sapiens with a cold-adapted Neanderthal, but the trends are still present when comparing modern and archaic humans within Africa. Overall, a modern Homo sapiens postcranial skeleton has thinner bones, smoother features, and more slender shapes. A slender frame is adapted for the efficient long-distance running ability that started with H. erectus. Furthermore, slenderness is a genetic adaptation for cooling an active body in hotter climates, which aligns with the ample evidence that Africa was the home continent of our species.
Aside from physical differences in the skeleton, researchers have also uncovered evidence of behavioral changes associated with increased cultural complexity from archaic to what they refer to as “behaviorally modern humans.” How did cultural complexity develop? Archaeology tells us much about the behavioral complexity of past humans by interpreting the significance of material culture (the things we make). In terms of advanced culture, items created with an artistic element, or as decoration, speak of abstract thought processes. The repeated demonstration of difficult artistic techniques and technological complexity over time indicates social learning and cooperation as well.
The interpretation of brain anatomy is another promising approach to studying the evolution of human behavior. Since the sheer size of the brain is not useful for weighing intelligence within a species, paleoanthropologists are instead investigating the differences in certain brain structures. The differences in organization between modern H. sapiens brains and archaic H. sapiens brains may reflect different cognitive priorities that account for modern human culture. As with the archaeological approach, new discoveries will refine what we know about the human brain and apply that knowledge to studying the distant past.
Taken together, the cognitive abilities in modern humans may have translated into an adept use of tools to enhance survival. Researchers Patrick Roberts and Brian A. Stewart (2018) call this concept the generalist-specialist niche: our species is an expert at living in a wide array of environments, with populations culturally specializing in their own particular surroundings. The next chapter tracks how far around the world these skeletal and behavioral traits have taken us.
