Australopithecus afarensis

Lucy

Australopithecus afarensis
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Primates Family: Hominidae Genus: Australopithecus Species: A. afarensis

Australopithecus afarensis is an extinct hominid species, which to some, is considered to be the "missing link" in human evolution. This is because the species shares a significant amount of traits with both chimpanzees and anatomically modern humans. Although A. afarensis is an older species than A. africanus, it is thought to be one of the closest ancestors to the genus Homo.

The monumental remains known as "Lucy" was unearthed from one of the most famous paleoanthropological finds in history. The potassium-argon dating found that the ancient species is thought to have lived between 3.76 and 2.9 million years ago with Lucy's remains dating to around 3.2 million years ago. The remains of A. afarensis remains so significant because it shows clear evidence that humans were bipedal before developing larger brains; opposed to the previously widespread assumption that human brains evolved before walking upright. The number of traits that it shares with both chimpanzees and anatomically modern humans (or Homo sapiens) is also very intriguing to many. The claim of discovering the potential missing link as well as the precise name of the species remains the subject of heated discussions within many scholarly circles to this date.

Distinguished Digs

AL 129-1

1973: Knee joint Kada Hadar, Ethiopia

Discovered in Hadar, Ethiopia by Donald Johanson, the angle of the proximal tibia and distal femur suggests a bipedal hominid. In addition, the bicondylar angle, deep patellar groove and lateral lip of the patellar groove suggest that it is in fact a hominid. A letter from Donald Johanson shows insight into how quickly he knew that the hominid walked upright: "In November 1973, during my first major expedition to Hadar, I found a perfectly preserved knee joint (minus the kneecap) at a locality numbered A.L. 128/129. All detailed anatomical analyses and biomechanical considerations of this joint indicate that the hominid possessing it, Australopithecus afarensis, was fully capable of upright bipedal posture and gait"

AL 129-1 Knee joint.

The Hadar site consists of four general members: The Basal member dates older than 3.4 million years old while the Sidi Hakum member dates to 3.22ma and is thought to be a woodland environment. The Denen Dora member dates to 3.18ma and is thought to have consisted of a forested/swampy area while the top layer is named Kada Hadar and dates to 2.3ma. The Kada Hadar member is much more spacious than the bottom layers.

AL 288-1

1974: Lucy Kada Hadar, Ethiopia

The Lucy find was a singular find and relatively complete (around 40%) compared to other hominid excavations in the surrounding area. Discovered by the International Afar Research Expedition (IARE), Lucy became one of the most notable finds in the history of human biological evolution. Lucy's remains dated to just under 3.18 million years old. The great significance of this find is mainly due to the fact that it was the first time there existed good evidence that humans were bipedal before developing larger brains.

In 1975, Michael Bush, one of Johanson's students, found the remains of more than thirteen A. afarensis individuals buried together which seemed to be the result of a flash flood or another natural disaster. This was assumed because of the presence of a catastrophic profile rather than an attritional profile. The catastrophic profile meaning that a group of organisms were buried in close proximity because of a catastrophe rather than time-related attrition.

The find yielded vital information about the social organization of A. afarensis: "It is clearly a mixed sample of young and old, large and small - meaning several females and several males. It looks very much like the composition of A. afarensis groups was like what we see in chimpanzees," Johanson explains.

Laetoli

1978: Laetoli Site: Footprints / Holotype Tanzania

The Laetoli site is located in Tanzania and is just south of Olduvai gorge. The site was being excavated by Mary Leakey and her team in 1975 when thirteen specimens of Australopithecus afarensis were discovered, including the current holotype of Australopithecus afarensis (a mandible). When returning to the site in 1978, the team uncovered over 20,000 animal tracks which included hominid footprints. The cluster of footprints found in the tuff dates from 3.76 to 3.49 million years ago. It should be noted that a number of scholars, with Mary Leakey in particular, would prefer the official name of this species to be called Praeanthropus afarensis.

DIK-1/1

1999: Lucy's Baby Dikika, Ethiopia

Selam (Lucy's Baby.)
© Copyright by Zeresenay Alemseged/ ARCH

This was the first major find by an African scientist. Zeray Alemseged was responsible for this find in Ethiopia and is equally astonishing as Lucy. This find was one of the most complete skeletons ever found, which included a scapula, and dated to around 3.31-3.35 million years old. The thyroid bone in the throat was not even fused yet which suggested a very early age of maturation (thus, the name Lucy's baby).

Like an adult A. afarensis, the Dikika baby had long, curved fingers. This suggests a good amount of time spent in trees even though there exists a great number of bipedal traits. The fossil also brings new data to the debate in the form of two shoulder blades, (or scapulae): bones previously unknown for this species. According to Alemseged, the shoulder blades of the child look most like those of a gorilla. The upward-facing shoulder socket is particularly apelike, contrasting sharply with the laterally facing socket modern humans possess. "This," Alemseged says, "may indicate that the individual was raising its hands above its head- something primates do when they climb."

Morphology

Cranial

Although the cranium is more primitive than the Australopithecus africanus, it remains classified as a gracile Australopith instead of a chimpanzee. The hominid has a relatively small brain with an average cranial capacity of 434cc. The brain of A. afarensis was about one-third the size of the average modern human brain, or about the same size as a modern ape's brain. It has an encephalization quotient of 2.5 and is quite chimpanzee-like when looking at it from behind.

The compound temporal neutral crests are responsible for a smaller brain capacity and its face is much more prognathus when compared to the Australopithecus africanus. Males also typically had large crests (called sagital crests) on top of their skulls while females did not. The sagital crests are from excessive chewing pressure from enlarged jaws and molars.

Although there are many apelike morphologies, there are also a number of human-like characteristics. A. afarensis provides the first evidence that australopiths retained a generally apelike skeletal design and body shape with the exception of lower limb features which are related to bipedalism. The posterior position of the foramen magnum also suggests bipedalsim. (McHenry,1991)

Dental

The canines show similar wear patterns as humans which may show a possible link between humans and Australopiths. Although the canines are a bit smaller than chimpanzees, they are bigger than other Australopiths which suggests dimorphism. There is minimal metaconid development and the cusps retain an asymmetric shape which is more chimpanzee-like. The megadontia quotient is around 1.7 and they have a shallow palate which is human-like. Although there are human-like characteristics,the tooth rows are more parallel and narrow which is more ape-like. These discrepancies show a possible link between human-like morphologies and ape-like characteristics which is both exciting and frustrating in the attempt to find a definitive answer to human evolution.

Australopithecus afarensis jaw & cranium.

The main differences between A. anamensis and A. afarensis relates to mandibular morphology and details of the dentition. The mandibular symphysis of A. anamensis is steeply-sloping compared with the more vertical symphysis of later hominids, including A. afarensis. In some respects the teeth of A. anamensis are more primitive than those of A. afarensis (e.g. asymmetry of the premolar crowns, less posteriorlyinclined canine root, and the relatively simple crowns of the deciduous first mandibular molars), but in others (e.g. the low cross-sectional profiles, and bulging sides of the molar crowns) they show similarities to more derived, and temporally much later, Paranthropus taxa. Compared with A. afarensis, A. anamensis also exhibits a primitive, horizontal tympanic plate.

“The large premolars of A. afarensis suggests they were frugivores, and the thick enamel on the teeth suggests they may have eaten nuts, grains, or hard fruit pies”.

Body size

Males average a weight of 100 lbs while females average a weight of 64lbs. This suggests that the average female weighed around 64% of the typical male. This indicates a significant amount of sexual dimorphism. Some studies have suggested that there exists such a great deal of dimorphism that it could in fact be two different species. But this has been refuted because although body sizes differ, the morphological features are continuous.

In July researchers reported that A. africanus actually had more apelike body proportions than its presumed ancestor, A. afarensis... McHenry et al suggests that: "The larger forelimbs of africanus might be an adaptation to a more arboreal life...But the hind limb is adapted to bipedality, and so there's no doubt that they were terrestrial. They probably spent a lot of time in trees--feeding, sleeping, or for protection" ...In afarensis, the apish proportions were not so pronounced--in all probability the presumed ancestor spent less time in the trees than the purported descendant.

Vertebrae

The vertebrae tend to have long, apelike spinous and transverse processes, and the vertebral bodies are intermediate in size compared with the ape and human conditions. Lumbar vertebrae are wedged such that the anterior length of the body is greater than the posterior length. The upper limb of A. afarensis is shorter than a great ape of comparable mass, but long relative to humans. (Jungers, 1994).

Ward and Bruce Latimer of the Cleveland Museum of Natural History analyzed the anatomy of spinal columns from the 3.2-million-year-old Australopithecus afarensis skeleton known as Lucy and a pair of roughly 2.5-million-year-old Australopithecus africanus specimens. The spines of the three australopithecines bend inward at the middle of the back and curve outward at the lower back just as in modern day humans. A bony column angled in this fashion positions the torso directly over the hip joints, fostering erect posture and a two-legged gait, Ward says. The shapes of australopithecine vertebrae also correspond closely to those of modern humans, the researchers found... In laboratory studies,Karen Steudel-Numbers of the University of Wisconsin-Madison and her team found that short-legged people consume substantially more oxygen while walking or running than long-legged people do. Steudel-Numbers estimates that australopithecines required an average of 30 percent more energy to walk a given distance than people do today.

Pelvis & knee joint: Bipedal locomotion

The average Homo sapien cranial capacity is around 1300-1500cc while the average A. afarensis cranial capacity is around 400cc, yet A. afarensis walked upright on two legs like humans. The evidence for bipedalism in A. afarensis is of great interest because the A. afarensis remains can be analyzed to infer that bipedal locomotion evolved before a larger brain.

Evidence for bipedalism is seen when examining the pelvis structure, knee joint and foramen magnum. The bones are strong and the pelvis is very human-like. When examining the bottom of the foot; there exists a non-opposable hallux (straight big-toe)with a longitudinal arch (for a springy foot). Although a lot of these features indicate a human-like biped, they still posses long arms, short legs and very curved fingers, which may indicate a significant amount of time in trees.

Apelike morphology includes the coronal orientation of the iliac blades, a somewhat long ischium without a raised tuberosity, a reduced acetabular anterior horn, and evidence of weakly developed sacroiliac ligaments. However, the pelvis shares with humans a short, wide ilium, a well developed sciatic notch and anterior inferior iliac spine, and wide sacrum. The femoral head and acetabulum, as well as sacroiliac and lower intervertebral joints, are small relative to humans of comparable size (Jungers, 1988). This evidence for the locomotion of A. afarensis is complemented by the discovery, at Laetoli, of several trails of fossil footprints (Leakey & Hay, 1979).

The team responsible for the theory that human ancestors learned to walk while still living in trees was led by Robin Crompton at Liverpool University and Susannah Thorpe at Birmingham University. The team's findings challenge the previously popular belief that human ancestors learned how to walk shortly after the split from chimpanzees; instead the findings suggest that bipedalism's origins could date to as far back as 24 million years ago.

The team claims that our tree-dwelling ancestors learned to walk on two feet because it helped them edge along outer branches while having their hands free to grasp ripe fruit. The tactic also enabled them to clamber between neighboring trees without having to descend to the forest floor.

Orangutan standing (balancing).

The scientists reached their conclusions after spending a year observing the movements of orangutans in Sumatra. The great apes of the region are the only species to spend their entire lives in the trees. Footage of nearly 3,000 movements showed the apes consistently walked on two legs to reach the outer branches of trees, using their arms primarily for balance.

Although this theory is new and intriguing, other theories about how bipedalism came to be are widely discussed and debated to this day. Some other popular bipedal origin theories include a thermoregulatory model proposed by Dr. Peter Wheeler, a postural feeding hypothesis proposed by Dr. Kevin Hunt and a provisioning model proposed by Owen C. Lovejoy.

Social Structure

Australopithecus afarensis had very curved finger bones which are relatively malleable throughout an organism's life. From this fact, paleoanthropologists can infer that they probably spent a significant amount of time in trees. Some have argued that they may have spent the night in trees to avoid terrestrial predation.

A more arboreal lifestyle is confirmed by the fact that A. afarensis has a different inner ear anatomy than anatomically modern humans: "Australopithecines are more similar to chimpanzees than to modern humans in their inner-ear anatomy," asserts C. Fred Spoor of the University of Liverpool in England; "This supports the view that australopithecines combined arboreal and terrestrial movement. Although humans and apes share many aspects of inner-ear anatomy, humans display markedly larger semicircular canals relative to body weight, Spoor holds. These structures support a balanced, upright stance, he says." This deduction of a more arboreal lifestyle not only suggests that bipedalism has some origins in the trees but also implies some crucial information about their day to day lifestyle.

The sexual dimorphism also yields information about their social characteristics. The moderate skeletal dimorphism of A. afarensis (greater than Pan and less than Gorilla) suggests a somewhat longer developmental period in males compared with females and is therefore inconsistent with a chimpanzee-like territorial strategy. At the same time, it is also markedly inconsistent with strategies like those of gorillas and orangutans, in which skeletal dimorphism is much more pronounced. Therefore, the co occurrence of moderate skeletal dimorphism, such as that found in modern humans and A. afarensis, and a reduced male canine is fully consistent with a pair-bonded reproductive strategy in early hominids; that is, if their reproductive strategy was chimpanzee-like, hominids should show only minimal skeletal dimorphism, or if it was orangutan, or gorilla-like, they should show greater skeletal dimorphism.

Canine dimorphism should be present in either case. Early hominid skeletal dimorphism is consistent with another special hominid character, the failure of male canine eruption to be delayed and thereby coincident with somatic maturation (as it is in all other hominoid species).
Thus, observed levels of body size dimorphism in A. afarensis do not imply that monogamy is any less probable than polygyny as the fundamental social system of these early hominids."

John S. Murphy 15:49, 1 May 2008 (CDT)

External Links

1 Anthro Daily
2 AL 129-1
2 Lucy's Baby