Chapter 8 Hominid Origins

Human Evolution Human evolution, beginning with our primate ancestors and ending with modern humans, involved many evolutionary changes that occurred at different times.

Biocultural Evolution All aspects of human adaptation, including technology, traditions, language, religion, marriage patterns, and social roles. Culture is a set of learned behaviors; it is transmitted from one generation to the next through learning and not by biological or genetic means. Material culture is part of the cultural complex

Paleoanthropology Paleoanthropology is defined as the study of early humans. Paleoanthropologists reconstruct the anatomy, behavior, and ecology of our ancestors: It is a diverse multidisciplinary pursuit seeking to reconstruct every bit of information possible concerning the dating, anatomy, behavior, and ecology of our hominin ancestors. Locate early hominin sites, collect faunal remains and artifacts

“How old are humans?” This question has a different answer depending on what criteria are used: Anatomically similar to modern humans: c. 200-130 kya. All members of Homo with stone tool use: c. 2.5 mya. All bipedial hominids: c. 6 mya. This chapter only deals w/hominid evolution before the genus of Homo, so 6-2 mya.

Fig. 10.1: Simplified summary of hominid evolution emphasizing major evolutionary events.

Overview of Human Evolution The hominid line split from the African apes about 6 mya when they became bipedal. Larger brains evolved millions of years after bipedalism. There are several fossil candidates for earliest known hominid, dating about 6 mya.

What Makes Us Human? Knowing that bipedalism emerged in the human lineage well before increased brain size, has your concept of what a human is changed?

Overview of Human Evolution By 3 million years ago, two distinct lines of hominid evolution diverged. One line led to several species known as robust australopithecines with large jaws for eating hard-to-eat foods The other line began to rely more and more on learned behavior, possibly including the first stone tools.

Overview of Human Evolution The genus Homo evolved from one of the australopithecine species sometime between 2.5 and 2 mya. Homo erectus appears in Africa by 2 mya and was the first hominid to expand out of Africa into Europe and Asia.

Overview of Human Evolution Brain size increased rapidly by 700 kya years ago. By 250 kya it was about the size of ours. But it’s not clear if these archaic humans were our direct ancestors or if we descended from another Homo species.

Overview of Human Evolution Around 130 kya anatomically modern humans appeared in Africa. The relationship btw the archaics and these hominids is unclear. By 50 kya, a “creative explosion” of culture took place, including new tool technologies and behaviors, such as art.

The First Hominids The first hominids evolved in Africa. They were adapted to woodland savannas and tropic grasslands. In such environments, bipedalism would have advantageous for travel to find food.

Differentiating Features Pelvis, scapula, and arm bones: shape and length. Face: protruding or reduced. Teeth: small or large canines; molar cusps. Skull: smaller or larger cranial capacity. Brow ridges and sagittal crest.

Australopithecus afarensis with brow ridges and protruding face. Australopithecus aethiopicus with a sagittal crest and protruding face. Neanderthal with brow ridges and more reduced face.

Sahelanthropus tchadensis 7-6 mya, Chad, Africa. Small apelike skull/braincase, but more receded/vertical face like later hominids Base of skull suggests it may have been bipedal.

Sahelanthropus Several hominid-like dental traits, including small canines. Huge browridge Possibly an early hominid.

Orrorin tugenesis 6 mya, Kenya, Africa. Leg remains suggests bipedal, but arm remains suggest somewhat arboreal. Molars suggest a hominid: small and humanlike rather than apelike.

Orrorin tugenensis

Ardipithecus ramidus (“Ardi”) 5.8-4.4 mya, Ethiopia, Africa. Teeth are ape-like, including large canines. Skull is mixed- small (apelike), but foramen magnum (large hole at base of skull) suggests bipedal.

Ardipithecus Pelvis shows derived characteristics Divergent big toe Woodland environment

Ardipithecus ramidus

Evolutionary Relationships Sahelanthropus, Orrorin, and Ardipithecus all date to around 6 mya. They all have some primate and some hominid features. This is expected for early evidence for when the hominid line first split from the ancient apes.

Evolutionary Relationships It is not yet clear if any or all of these were true hominids. However, the evidence is new and still under study. The relationships among them and the possible relationships to later hominids are unclear.

Australopiths (4.2–1.2 mya) The best-known, most widely distributed, and most diverse of the early African hominins are colloquially called australopiths. This group of hominins is made up of two closely related genera: Australopithecus and Paranthropus. These hominins have an established time range of over 3 million years, stretching back as early as 4.2 mya and not becoming extinct until apparently close to 1 mya.

Australopithecus Genus Lived 4.2 to 1.4 mya in Africa. Characterized by: clearly bipedal (but not necessarily identical to Homo in this regard). relatively small brain size. large and protruding faces. large teeth, especially back molars.

Australopithecus anamensis East Africa, 4.2-3.9 mya. 52 specimens have been found. Clearly hominid based on leg remains. But many apelike skull, jaw, and dental features, such as large canines and small ear hole. Possibly a link between the earliest and later hominids.

Australopithecus anamensis

Australopithecus afarensis (“Lucy”) East Africa, 4-3 mya. Clearly bipedal evidence (bones and footprints), including knee joint, non-divergent big toe, heel stride, and well-developed arch. However, it may have spent some time in trees. Some apelike features, especially small brain and protruding lower face.

Australopithecus afarensis Teeth and jaws are between ape and human. Lucy: 40% complete skeleton of a small (3’3”, 60 lb) adult female. A likely link between the earliest and later hominids. The level of sexual dimorphism seems to be about the same as seen among modern humans.

Australopithecus afarensis

Infant A. afarensis Skeleton An important new find of a mostly complete female infant A. afarensis skeleton was announced in 2006. The discovery was made at the Dikika locale in northeastern Ethiopia, near the Hadar sites. The infant comes from the same geological horizon as Hadar, dating 3.3 mya.

Kenyanthropus platyops East Africa, 3.5-3.2 mya. It has a combination of primitive and more humanlike features. Primitive features include a small brain, jutting lower jaw, small ear hole. Humanlike features include small molars, a flat face (contested), and tall cheek region.

Kenyanthropus platyops It is not yet clear if it falls within the Australopithecus genus.

The First Hominids Fig. 10.2: Location of some of the major sites in Africa where early hominid specimens have been found.

Robust Australopithecines A. aethiopicus, A. robustus, A. boisei (now often termed Paranthropus boisei). 2.5-1.4 mya in Africa. Relatives, but not direct ancestors Robust back teeth, jaws, and faces. Indicates diverse diet w/hard to chew foods. A. aethiopicus

Robust Australopithecines Teeth structures, though, are humanlike but larger. Skulls also show heavy chewing. Overall body size was more humanlike. Brain size was small. No tool associations, although A. robustus could have made them.

Robust A. boisei

Australopithicus africanus 3.3-2.5 mya, South Africa. Possible ancestral hominid. Reduced canines, large face, small brain but no sagittal crest like the robust forms. Considered a descendant of A. afarensis.

Australopithicus garhi 2.5 mya, East Africa. Possible ancestral hominid. Small brain. Features similar to A. afarensis, front and back teeth are large, but not like the robust species. Faunal remains with stone tool cut marks suggest that this was a toolmaker.

Australopithicus garhi

Evolutionary Relationships Science is cumulative. Our ideas change as new evidence is included and studied. New species and subspecies are being found all the time (e.g. H. floresiensis), meaning that our interpretations of the data change.

Evolutionary Relationships At present, the overall data suggest … The 1st hominids date back to c. 6 mya. There was much diversity, with many species going extinct. This diversity increased 3 mya, with several robust and nonrobust species.

Evolutionary Relationships At least one of the nonrobust species with early anatomical and behavioral changes more like Homo was the ancestor of modern humans.

Cladistic Family Trees We try to determine the relationships among the earlier hominids. 1st step is to look at the dates in order to understand their relationships across time (Fig. 10.26). 2nd step is to look at similar traits and forms that share derived characteristics not found in the other species.

Evolutionary Trends (Fig. 10.26)

Unanswered Questions Which early species led to later hominids- Sahelanthropus, Orrorin, Ardipithecus, or none of these? Did all the robust species have a common ancestor or evolve independently? Which early hominid(s) evolved into the genus Homo- A. africanus, A. garhi, or an undiscovered species?

Cladistic Family Trees This is complicated by the apparent frequency of homoplasy (similarities due to independent evolution). Our current ‘family trees’ are summaries of alternative hypotheses and may change as new data is added (Fig. 10.27).

Evolutionary Trends (Fig. 10.27)

Bipedalism: Savanna The first fossil evidence (c. 4.4-2 mya) came from what were grasslands (savanna) and mixed grassland/woodland environments. Early explanations assumed bipedalism would have been an advantage in finding harder-to-find food, traveling long distances, dealing with hotter temperatures, and escaping predators.

Bipedalism: Savanna However, this hypothesis is challenged because more recent evidence dating to before 4.4 mya (Sahelanthropus, Orrorin, and Ardipithecus) came from environments that were relatively wet forests and woodlands.

Bipedalism: Tool Use Darwin hypothesized that bipedalism allowed them to carry stone tools. This was expanded in the ‘60s: as stone tool use increased, NS favored enhanced learning, intelligence, & larger brains; longer infancy and child dependency evolved with larger brains, making tool use even more important.

Bipedalism: Tool Use Tool use led to larger brains, which led to more reliance on tool use. Recent evidence, however, shows bipedalism evolved as much as 3.5 my before larger brains or stone tool use. There may be limited merit as they (like chimps) could have used simple, perishable organic tools.

The Environmental Context: Lovejoy and Infants Lovejoy proposed in the 1980s that bipedalism evolved to increase survival of infants and other dependant offspring. Remember that apes and hominids have a long period of infant dependency, causing slow rates of pop growth.

Bipedalism: Lovejoy and Infants In this model, it was selected for because it allowed some to forage for food and carry it back to the rest. This would have allowed for increased survival of offspring and population growth. More infants could be cared for at the same time.

Bipedalism: Predator Avoidance and Heat Stress It allowed them to see farther and above tall grass on the savanna. This would help them avoid large carnivores. Heat stress It reduced exposure to UV and increases cooling by air movement. This would help with heat on the savannas.

Bipedalism: Predator Avoidance and Heat Stress However, recent evidence suggests that the earliest hominids lived in woodland environments. This means that predator avoidance and heat stress may not have been why bipedalism first evolved.

Bipedalism: Procuring Food and Energy Efficiency It is more efficient for long-distance travel when looking for, gathering, and carrying food. However, it does not appear that the earliest hominids lived in savanna environments.

Bipedalism: Procuring Food and Energy Efficiency On the other hand, climatic data shows that woodlands and forests were shrinking, meaning the hominids would have had to travel between the clusters to find food.