Introduction to Biological Anthropology: Notes 21 Apes and early hominins Copyright Bruce Owen 2011 the first known hominoids (apes) appeared in the

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1 Introduction to Biological Anthropology: Notes 21 Apes and early hominins Copyright Bruce Owen 2011 the first known hominoids (apes) appeared in the late Oligocene, 27 mya example Oligocene ape: genus Proconsul (probably various species) lived in tropical rainforest fairly large, like a macaque pounds hominoid traits no tail larger body size slightly larger brain to body size ratio short forelimbs and narrow chest indicate they were quadrupedal, walking on top of branches, as many monkeys do rather than hanging by arms, as modern apes do teeth indicate frugivorous diet sturdy canines procumbent incisors thin enamel on the molars: relatively soft foods Y-5 molars create a broad pulverizing surface, rather than the pointy piercing or sawtoothed shredding surfaces useful for other diets another Oligocene hominoid: Morotopithecus bishopi similar to Proconsul, but with hints of more apelike posture and locomotion evidence: scapula (shoulder blade) suggests that it climbed and hung from branches, maybe brachiated these are traits that became common in later apes Hominoid radiation in the Miocene: the middle Miocene (15-10 mya) saw a great radiation of hominoids (apes) that is, the hominoids split into many different lines, with different species adapting to many different niches why? We don t really know, but: lots of climate changes in the Miocene from the middle Miocene on, it got cooler and drier tropical forests shrank, and there were greater areas of open woodland and savanna the climate also began to change back and forth between warmer and cooler more rapidly, on a scale of just tens of thousands of years this is too rapid to show in the general temperature chart in the slide maybe these rapid changes, rather than the climate itself, was the key maybe something about apes made them well suited to handle changing environments maybe the ability to get by on a range of different foods, rather than being strongly committed to just one category of foods also, Africa and Eurasia got close and joined, cutting off the Tethys sea around mya leading to an exchange of animals and plants

2 Intro to Biological Anthro F 2011 / Owen: Apes and early hominins p. 2 probably changing the ecology in many ways and allowing some species of early apes (genus Proconsul) to spread out of Africa to more varied environments so populations of apes in different environments, surrounded by new varieties of plants and animals, evolved into many different species of apes most of which went extinct by the end of the Miocene or slightly later the many, varied Miocene hominoids (Miocene apes) Kenyapithecus, Oreopithecus, Dryopithecus, Sivapithecus, Ramapithecus, etc. general trend towards more chewing -- eating harder or more fibrous foods presumably in response to the drying, more seasonal climate which would have encouraged woodier, tougher plants compared to the tropical rainforest Miocene hominoids (apes) had features for heavy chewing molars of some species had thick enamel, allowing for more wear and more pressure to be applied to break hard seeds molars had lower, rounded cusps, better for grinding, less designed for shearing leaves that is, less specialized, more generalized for a varied diet molars tend to be worn from lots of grinding U-shaped dental arcade typical of apes, different from hominids more space for larger temporal muscles (that pull the lower jaw up against the upper jaw) this is visible by looking at the space where the temporal muscle passes between the temporal bone of the brain case and the zygomatic arch this whole space is filled by the temporal muscle more massive mandible (lower jaw), to withstand the increased chewing forces at least two of the Miocene hominoids (Dryopithecus and Oreopithecus) were arboreal, adapted to hanging by their arms like quadrumanual or brachiating apes they have the classic anatomical adjustments to brachiation or quadrumanuality wide ribcage shoulder blades at the back, not on the sides long arms, short legs long, curved fingers this is the first definite appearance of these classically apelike features, which clearly led eventually to humans back in the late Oligocene, Morotopithecus may have had some or all of these traits already We don t know which modern apes (and humans) descended from which of the Miocene apes we may not have found the species that were ancestral to the various modern hominoids (humans, gorillas, chimps, orangs, gibbons, siamangs) except Sivapithecus, which looks to be the ancestor, or a relative of the ancestor, of orangutans

3 Intro to Biological Anthro F 2011 / Owen: Apes and early hominins p. 3 by the late Miocene and early Pliocene, the temperature was dropping rapidly tropical forests were shrinking leaving patches of temperate forest separated by increasingly open grassland as their forest environments shrank, many of Miocene apes went extinct in the Late Miocene only a few lineages survived leading to the modern apes, and to us there used to be many more kinds of apes in the world; now just a few remain By the end of the Miocene or start of the Pliocene, between 6 and 5 mya, one of the surviving ape lineages was starting to show signs of resembling us these were the first hominins -- the lineage that led specifically to us this surviving species of ape responded to these changes in a new way it came down from the trees and became habitually bipedal the features that had helped the Miocene apes live in trees also happened to allow for limited bipedalism on the ground much as modern apes (gorillas, chimps, gibbons, etc.) can walk bipedally at times one theory: as trees got farther apart and there were more open spaces, natural selection may have favored apes that were better at bipedal walking between the patches of trees but some of the early evidence of bipeds comes from areas that were forested, so the spreading savannah grasslands might not have been the key factor this was the split that led to hominins: the bipedal apes a subset of hominoids (apes) other than bipedalism, they weren't very different from other apes, especially at first humans are the only living species of hominins, but there were many more in the past the first thing that distinguished our ancestors from other apes was bipedalism not large brains; those came much later we have very few fossils from the late Miocene to tell us about the origin of hominins but DNA evidence shows that hominins split from the lineages that led to chimps, bonobos, and gorillas around 7 to 5 mya that is, we, chimps, bonobos, and gorillas all descended from one ancestral species of ape that lived near the end of the Miocene, around 7 to 5 mya we also can be pretty sure that this transition to bipedalism happened in Africa there were Miocene apes in both Africa and tropical Asia but all of the remains of hominins other than the much later remains of our genus Homo are from Africa First suspected hominin: Sahelanthropus tchadensis (7-6 mya) (Late Miocene) about when hominins split from the line that led to chimps some apelike features chimp-sized brain ape-like teeth (large canines, wide incisors, U-shaped dental arcade, etc.) key hominin feature foramen magnum under braincase: upright, bipedal?

4 Intro to Biological Anthro F 2011 / Owen: Apes and early hominins p. 4 some features not seen again until much later hominins fairly vertical face massive browridges did these continue directly to the later hominins, or did later hominins evolve these features independently later? But this one fossil is all we have, with no post-cranial (non-head) remains so we can t be sure about its bipedalism or relationship to other hominoids the late Miocene ape Orrorin tugenensis (6.1 to 5.2 mya) The head of its femur (thigh bone) is large, suggesting that it evolved under selection for durability in a creature that spent a lot of time on its hind legs a biped if correct, that would make Orrorin an early hominin Its humerus (upper arm bone) and one long, curved finger bone suggest it still had some features associated with hanging in trees The environment, as reconstructed from other animal fossils found with it, was open grassland with patches of forest just as expected for an arboreal ape that was becoming a part-time terrestrial biped while Orrorin was still around, Ardipithecus kadabba and its probable descendant species, Ardipithecus ramidus, appeared (5.8 to 4.4 mya) A. kadabba is still poorly known, based on teeth and a few larger fragments A. ramidus is now represented by a remarkable fossil that represents almost half of the whole skeleton nicknamed Ardi plus a few fragments of several other individuals still clearly an ape, with a roughly chimp-sized brain ( cc) the foramen magnum is more forward, under the head, than in other apes, hinting at habitual upright posture - and thus bipedalism pelvis shape suggests imperfect bipedalism more bowl-shaped to support internal organs when standing upright flaring shape positions upper leg muscles better for bipedalism but still has a grasping big toe and other features that suggest quadrupedal branch-top locomotion in trees apparently lived in wooded areas of a patchy woodland and grassland environment based on seeds and other animal fossils found with Ardipithecus fossils so bipedalism may have helped to move in the grassland parts of the environment between the forest patches or it may have provided some advantage within the forested areas, such as reaching higher foods while standing on the ground also notable for the males having canines that are only somewhat larger than in females much less sexual dimorphism than modern chimps or gorillas suggests less male-male competition than in chimps or gorillas (our closest living relatives), and presumably other differences in social behavior

5 Intro to Biological Anthro F 2011 / Owen: Apes and early hominins p. 5 thus maybe both bipedalism and some still poorly known changes in social behavior involving less male-male competition came before brain enlargement, tool making, and so on

Introduction to Biological Anthropology: Notes 20 Apes and early hominins Copyright Bruce Owen 2010 the first known hominoids (apes) appeared in the

Introduction to Biological Anthropology: Notes 20 Apes and early hominins Copyright Bruce Owen 2010 the first known hominoids (apes) appeared in the late Oligocene, 27 mya example Oligocene ape: genus