Question

Name and explain the function of at least four morphological traits of Ardipithecus ramidus and discuss the fossils position in the evolutionary tree of hominins

Anthropology

Answer 1

Ardipithecus ramidus :-

This species was a facultative biped and stood upright on the ground but could move on all four limbs in trees. Features of the anatomy are extremely primitive.

_Brain

about 300-350cc, similar in size to modern female chimpanzees and bonobos

Body size and shape

similar in size to modern chimpanzees. The most complete specimen, a female, stood about 120cm tall
males were only slightly larger than females
the body shape was more ape-like than humans, but differed from living African apes in a number of significant features

Limbs

• mix of primitive and derived features suggest this species was able to walk upright on the ground yet efficiently climb trees
• long powerful arms that were not used for weight-bearing or knuckle-walking as with quadrupedal apes
• bones in the wrist (particularly the midcarpal joint) provided flexibility and the palm bones were short. These features suggest this species was not a knuckle-walker and that the palms could support the body weight when moving along branches
• finger bones were long and curving, both features useful for grasping branches
• upper and lower legs bones (femur and tibia) have features consistent with bipedalism
• feet were relatively flat and lacked arches, indicating this species could probably not walk or run long distances
• they had grasping abducted toe characteristic of gorillas and chimps
• the foot was more rigid than chimpanzees with the bases of the four toe bones oriented to reinforce the forefoot when pushing off. Chimps have a highly flexible midfoot that improves their ability to grasp and climb but are less effective for propulsion when walking on ground

Pelvis

• has a mix of features useful for both climbing and upright walking and suggests the species still spent significant time in the trees
• shape of the upper blades (ilium) appear short and broad like Australopithecus afarensis, indicating that the gluteal muscles had been repositioned. This lowered the body's centre of mass so to balance on one leg when walking
• the lower pelvis is large and the angle of the ischial surface does not face upward as it does in humans and Australopithecus. These are primitive features that suggest this species had massive hindlimb muscles for climbing and did not walk like A. afarensis
• the sciatic knotch is similar in size and shape to later hominins. This is a derived feature and is not found in chimpanzees

Jaws and teeth

• much of the dentition is ape-like including relatively large canines and molars
• tooth enamel thickness is intermediate between that of chimpanzees and Australopithecus
• canines are less projecting and smaller than those of all other known apes and there is no evidence of honing. The base of the canines in both sexes are similar in size to female chimpanzees and male bonobos, but have shorter crown heights
• upper canines are shaped like diamonds, rather than the pointed shape seen in African apes, whch is a derived feature shared with Australopithecus afarensis . Lower canines appear to have less derived features.
• the jaw displayed significant forward projection compared to humans, but less than modern African apes
• pre-molars have derived features that are more advanced in the human direction
• canines (non-sharpened and small) and other teeth share features with Orrorin tugenensis

Skull

• skull rests atop the spinal column, indicating this species was bipedal, although it probably walked in a slightly different manner than humans
• the cranial base is short from front to back, indicating the head balanced on top of the spine
• the face is small and in a more vertical position than chimpanzees
• the ridge above the eye socket is unlike that of a chimpanzee

FOSSIL POSITION OF HOMININS:-

Darwin's great insight, and the unifying principle of biology today, is that all species are related to one another like sisters, cousins, and distant kin in a vast family tree of life. The implications are breathtaking; if we could travel back far enough in time, we would find common ancestors between ourselves and every other living organism, from porcupines to flamingoes to cactuses. Our immediate evolutionary family is comprised of the hominoids, the group of primates that includes the "lesser apes" (siamangs and gibbons) as well as the "great apes" (chimpanzees, bonobos, gorillas, and orangutans). Among the great apes, our closest relatives are the chimpanzees and bonobos (Figure 1). The fossil record, along with studies of human and ape DNA, indicate that humans shared a common ancestor with chimpanzees and bonobos sometime around 6 million years ago (mya). We begin this discussion of our species' evolution in Africa, near the end of the geological time period known as the Miocene, just before our lineage diverged from that of chimpanzees and bonobos.

The human evolutionary family tree:- (Figure1)
Relationships and estimated divergence times of the living apes are shown. Hominins are all species, including side-branches and extinct species, on the human line (highlighted) after our Last Common Ancestor with chimpanzees & bonobos (marked “A”). Some fossil hominin species are shown in the right-hand column, with their approximate age ranges indicated; early hominins: gray, Australopithecus: blue, Homo: orange.

Miocene Origins of the Hominin Lineage
In order to understand the evolution of any species, we must first establish its ancestral state: what sort of animal did it evolve from? For our lineage, this requires that we try and reconstruct the Last Common Ancestor of humans and chimpanzees (marked "A" in Figure 1). The Human-Chimpanzee Last Common Ancestor (HC-LCA) is the species from which the hominin lineage and the chimpanzee & bonobo lineage diverged. Hominins are species on our branch of the hominoid tree after the split with the chimpanzee & bonobo line, including all of the extinct species and evolutionary side branches (Figure 1).

There was a great diversity of ape species in the Miocene, with dozens of species known from the fossil record across Africa, Europe, and Asia. These species varied in their anatomy and ecology, and it is not clear which, if any, of the fossil species discovered thus far represent the HC-LCA (Kunimatsu et al. 2007; Young and MacLatchy, 2004). Nonetheless, we know from fossil and comparative evidence that it was much more similar to living apes than to living humans. The HC-LCA would have had an ape-sized brain and body, with relatively long arms and fingers and a grasping foot that allowed it to forage in the trees. The canine teeth were probably large and sharp, as seen in several Miocene hominoids. Moreover, the canines were probably sexually dimorphic, with males having much larger canines than females, as seen among the living great apes and Miocene fossils. Like living apes it would have walked quadrupedally (on all fours) when on the ground, and its diet would have consisted almost entirely of plant foods, primarily fruit and leaves.

Early Hominins
Changes from an ape-like anatomy are discernible in hominoid fossils from the late Miocene in Africa. Some hominoid species from this period exhibit traits that are typical of humans but are not seen in the other living apes, leading paleoanthropologists to infer that these fossils represent early members of the hominin lineage. The first human-like traits to appear in the hominin fossil record are bipedal walking and smaller, blunt canines.

The oldest hominins currently known are Sahelanthropus tchadensis from Chad (Brunet et al. 2005) and Orrorin tugenensis from Kenya (Senut et al. 2001). Sahelanthropus, dated to between 6 and 7 mya, is known from a largely complete skull and some other fragmentary remains. Its brain size, 360cc, is within the range seen in chimpanzees, and the skull has a massive brow ridge, similar in thickness to male gorillas (Brunet et al. 2005). However, the position and orientation of the foramen magnum, the hole in the base of the skull through which the spinal cord passes, suggests that Sahelanthropus stood and walked bipedally, with its spinal column held vertically as in modern humans rather than horizontally as in apes and other quadrupeds (Zollikofer et al. 2005). Orrorin is known primarily from postcranial fossils, including a partial femur. The proximal portion of the femur shows similarities to those of modern humans, suggesting the species was bipedal (Pickford et al. 2002). No skulls of Orrorin have been recovered, and so its cranial morphology and brain size are uncertain. In both Orrorin and Sahelanthropus the canine teeth of males are larger and more pointed than in modern humans, but are small and blunt compared to the canines of male apes. This suggests that canine sexual dimorphism — and by extension, competition among males for mating access to females — was diminished in these early hominins compared to the great apes.

By far the best known early hominin is Ardipithecus ramidus, a 4.4 million year old species from Ethiopia, which is known from a nearly complete skeleton as well as numerous other dental and skeletal remains (White et al. 2009). Ar. ramidus and an older, related species known from fragmentary remains, Ar. kadabba (5.8–5.2 mya), have reduced canines similar to those of Orrorin and Sahelanthropus. The skull of Ar. ramidus is rather ape-like and broadly similar to that of Sahelanthropus, with a small chimpanzee-sized brain of 300–350cc (Figure 2). The Ardipithecus postcranial skeleton is intriguing. Although badly fragmented, the pelvis recovered reveals a morphology quite different from that of living apes, with a shorter, more bowl-like shape that strongly suggests Ardipithecus walked bipedally; this is consistent with the foramen magnum position, which suggests an upright posture. However, its long forelimbs and fingers and its divergent, grasping first toe (hallux) suggest Ardipithecus spent much of its time in the trees. The overall impression is of a largely arboreal species that walked bipedally whenever it ventured to the ground.

Anatomical comparisons of apes, early hominins, Australopithecus, Homo erectus, and humans. (Figure 2)
A male chimpanzee skull is shown as an example of modern apes. Early hominins and Australopithecus retained ape-size brains. Bipedal walking evolved very early in the hominin lineage, but Ardipithecus (and possibly other early hominins) retained a grasping foot that may have diminished bipedal efficiency. Reduced canine size also evolved early in the hominin lineage, although early hominin canines were larger and more pointed than those of later hominins. Molar size increased with Australopithecus but later is reduced in Homo (note: premolars are not shown in this schematic). Arboreal locomotion, as indicated by the presence of long arms, curved fingers and toes, and other forelimb features, was common throughout much of the hominin lineage.