Australopithecus afarensis

This species is one of the best known of our ancestors due to a number of major discoveries including a set of fossil footprints and a fairly complete fossil skeleton of a female nicknamed 'Lucy'.

Background of discovery

Age

This species lived between 3.9 and 2.8 million years ago.

What the name means

Australopithecus means ‘southern ape’ and was originally developed for a species found in South Africa. This is the genus or group name and several closely related species now share this name.

The word afarensis is based on the location where some of the first fossils for this species were discovered – the Afar Depression in Ethiopia, Africa

Key fossil discoveries

During the 1970s, two fossil hunting teams began uncovering evidence of ancient human ancestors in east Africa. One team, co-led by Donald Johanson, was working at Hadar in Ethiopia. The other team led by Mary Leakey, was over 1,500 kilometres away at Laetoli in Tanzania. Fossils discovered at the two sites were found to have very similar features and ages but they did not match the fossils of any species known at that time. A new species name, Australopithecus afarensis, was therefore created for them in 1978.

This species is now represented by several hundred fossils from east Africa.

Important specimens

• ‘Lucy’ AL 288-1 – a partial skeleton discovered in 1974 by Donald Johanson in Hadar, Ethiopia. This relatively complete female skeleton, dated to 3.2 million years old, is the most famous individual from this species. She was nicknamed ‘Lucy’ after the song ‘Lucy in the sky with diamonds’ sung by The Beatles. This song was very popular at the time she was found. Lucy was only about 110 centimetres tall but was a fully grown adult when she died. She was bipedal which means she could walk on two legs but she probably also spent a lot of time climbing trees in search of food or shelter.
• Knee AL 129 1a + 1b discovered in 1973 in Hadar, Ethiopia. When this 3.4 million year old knee was discovered, it was the first fossil to provide evidence that our ancestors that had been walking on two legs for more than three million years. The discovery of this fossil encouraged Donald Johanson’s team to return to Hadar where they ultimately found ‘Lucy’ and hundreds of other fossils of this species.
• LH 4 – a lower jaw discovered in 1974 by Mary Leakey’s team in Laetoli, Tanzania. This fossil is the type specimen’or official representative of this species.
• The ‘First Family’ - a group of A. afarensis fossils discovered in Hadar, Ethiopia in 1975. It became known as the ‘First Family’ because it contained fossils from nine adults and four children. Some unknown disaster overcame this family group, burying them all at the same time. This important find has allowed scientists to gather insights into the biology and development of a single fossil species. It also provided evidence that this species lived in small groups based on possible family bonds.
• ‘Selam’ or ‘Lucy’s baby' – a partial juvenile skeleton discovered in 2000 in Dikika, Hadar, in Ethiopia. She lived 3.3 million years ago and was about 3 years old when she died. Bones are preserved that have never before been found in this species. This is the earliest juvenile hominin skeleton ever found and should provide fantastic opportunities to uncover more about this species and about how our early ancestors developed.
• 'Lucy's big brother', nicknamed Kadanuumuu ('big man' in Afar) - a partial skeleton of a male uncovered in Afar, Ethiopia. An arm bone was first found in 2005 and other parts recovered over the next four years included shoulder blade, ribs, neck vertebra, pelvis, leg bones (complete tibia and partial femur) and a collarbone. This individual stood about 1.6 metres tall (30% larger than 'Lucy') and lived about 3.6 million years ago.

Laetoli footprints

These fossil footprints were discovered in Tanzania, East Africa and date to 3.6 million years ago. Fossil bones from A. afarensis have been found nearby so it is presumed that they left the tracks.

The quite human-like footprints were made by hominins that walked through a layer of ash burst that had settled on the ground after a distant volcano erupted. Raindrops made the ash damp and, even now, the indentations made by these raindrops can still be seen on parts of the ash layer. At first glance, it looks like two people walked side-by-side. The one on the left was much smaller than the other and may have been a child. On closer examination, we can see that the prints on the right-hand side are blurred and were actually made by two adults - one following the other and treading into the prints left by the first. Part way along the trail the individuals pause and turn to the left before continuing. The sun soon dried the damp ash, which hardened like cement. The active volcano continued to throw out ash until a layer up to 20 centimetres thick blanketed both the ground and the footprints.

Over millions of years, additional sediments were deposited and some were eroded away by wind and water. Eventually some of the footsteps lay uncovered. Then a lucky fossil hunter came across them and the story began to be revealed.

The footprints are of major significance as they are the first direct evidence (ie not fossils bones) that our ancestors were walking upright by 3.6 million years ago. The fossil footprints are very similar to our own footprints. They show that the heel was the first part of the foot to strike the ground. The big toe was aligned with the other toes and left a deep impression showing that each step ended with the toe pushing downwards. The feet also had central arches to help launch the body into each step.

Distribution

Fossils have been found at Hadar in Ethiopia and Laetoli in Tanzania, about 1,500 kilometres away.

Relationships with other species

Australopithecus afarensis is usually considered to be a direct ancestor of humans. It is also considered to be a direct ancestor of later species of Australopithecus and all species in the Paranthropus genus.

The names Praeanthropus africanus and Praeanthropus afarensis have been suggested as alternatives by researchers who believe this species does not belong in the genus Australopithecus.

Contemporaries?

In 2015, a team under Yohannes Haile-Selassie described in the journal Nature a new species A. deyiremeda (from the Afar language, deyi meaning ‘close’ and remeda meaning ‘relative’). The fossils date to 3.5 to 3.3 million years old and were discovered in Woranso-Mille in Ethiopia, close to sites of a similar age that produced A. afarensis specimens. If correct, A. afarensis was not the only hominin around in east Africa at this time.

The fossils, all found in March 2011, include a partial upper jaw bone (holotype BRT-VP-3/1), two lower jaws (paratypes BRT-VP-3/14 and WYT-VP-2/10) and an isolated P4 tooth in a maxillary fragment (referred specimen BRT-VP-3/37). Key features included forward cheek bones, three-rooted premolars and small first-molar crowns. Comparisons were made with other known middle Pliocene hominins such as Kenyanthropus platyops and A. afarensis; the discovers believed there were enough differences to warrant a new species designation. Others disagree, claiming that making comparisons with K. platyops is problematic (the only skull was extremely distorted and possibly badly reconstructed) or that the small sample size is not enough to draw such major conclusions. They consider the remains part of a variable A. afarensis population instead.

Whether these particular fossils do represent a new species or not, it is becoming likely that A. afarensis was not the only species around at this time in this area. Haile-Selassie announced in 2012 the discovery of a 3.4-million-year old partial foot (BRT-VP-2/73), found in the Afar Region of Ethiopia. It clearly did not belong to A. afarensis, but has yet to be assigned to a species.

Key physical features

Fossils show this species was bipedal (able to walk on two legs) but still retained many ape-like features including adaptations for tree climbing, a small brain, and a long jaw.

Body size and shape

• females grew to only a little over one metre in height (105 – 110 centimetres) and males were much larger at about 150 centimetres in height
• rib cage was cone-shaped like those of apes

Brain

• brain was small, averaging approximately 430 cubic centimetres and comprised about 1.3% of their body weight
• reorganisation of the brain may have begun with some enlargement to parts of the cerebral cortex

Skull

• many cranial features were quite ape-like, including a low, sloping forehead, a projecting face, and prominent brow ridges above the eyes.
• unlike most modern apes, this species did not have a deep groove lying behind its brow ridge and the spinal cord emerged from the central part of the skull base rather than from the back.
• males had a bony ridge (a sagittal crest) on top of their skull for the attachment of enormous jaw muscles. In this species, the crest was very short and located toward the rear of the skull.
• a small hyoid bone (which helps anchor the tongue and voice box) found in a juvenile specimen suggests A. afarensis had a chimp-like voice box
• semi-circular ear canal similar in shape to African apes and A. africanus, suggesting this species was not as fast or agile on two legs as modern humans

Jaws and teeth

• jaws and teeth were intermediate between those of humans and apes:
• jaws were relatively long and narrow. In the lower jaw, the teeth were arranged in rows that were slightly wider apart at the back than at the front. In the upper jaw, the placement of the last molar results in tooth rows that curve in at the back.
• front incisor teeth were quite wide.
• canine teeth were pointed and were longer than the other teeth. Canine size was intermediate between that of apes and humans. Like apes, males had much larger canines than females.
• a gap (diastema) was often present between the canines and adjacent teeth. This ape-like feature occurred between the canines and incisors in the upper jaw, and between the canines and premolars of the lower jaw.
• premolar teeth in the lower jaw had ape-like cusps (bumps on the chewing surface). The front premolar tended to have one large cusp (ape-like) rather than two equal-sized cusps as in humans.
• back molar teeth were moderate in size and were human-like in having a ‘Y-5’ pattern. That is, they had five cusps arranged so that the grooves between the cusps form a Y-shape.

Pelvis

• pelvis was human-like as it was short and wide, but it lacked the refinements that enable humans to walk with a striding gait

Limbs

• limbs displayed human-like features that indicate an ability to walk on two legs
• femurs (thigh bones) that slanted in toward the knee
• knees with enlarged and strengthened outer condyles
• arched feet and wide heels
• big toes aligned with the other toes and not opposable
• ape-like features that suggest an ability to climb trees
• powerful arms with long forearms
• very short thigh bones
• long, curved finger and toe bones.
• shoulder blade socket that faces upwards like an ape’s, rather than to the side like a human’s, but shared other similarities with human shoulder blades

Lifestyle

Culture

This species probably used simple tools that may have included sticks and other non-durable plant materials found in the immediate surroundings. Stones may also have been used as tools, but there is no evidence that stones were shaped or modified in any way. It seems likely that they lived in small social groups containing a mixture of males and females, children and adults. Females were much smaller than males.

In 2010, fossil bones bearing cut marks were found in Dikika in Ethiopia, dating to about 3.4 million years old. These bones show clear evidence of stone tools being used to remove flesh and to possibly smash bone in order to obtain marrow. No actual tools were found so it is not known whether the 'tools' were deliberately modified or just usefully-shaped stones. Although no hominin remains were found at the site, the discoverers believe A. afarensis was responsible for the cut marks as no other hominin species dating to this period have been found in this region.

Environment and diet

This species occupied a range of environments. Some populations lived in savannah or sparse woodland, others lived in denser forests beside lakes. Analysis of their teeth, skull and body shape indicates a diet that consisted mainly of plants. However, fossil animal bones with cut marks found in Dikika in 2010 have been attributed to this species, suggesting they may have included significant amounts of meat in their diets. Microscopic analysis of their tooth enamel shows that they mostly ate fruits and leaves rather than seeds and other hard plant material. Their cone-shaped rib cage indicates they had large bellies adapted to a relatively low quality and high bulk diet. The position of the sagittal crest toward the back of the skull indicates that the front teeth processed most of the food.

Further reading:

Yohannes Haile-Selassie et al (2015) ‘New species from Ethiopia further expands Middle hominin diversity’, Nature 521, 483-488

Yohannes Haile-Selassie et al (2012) ‘A new hominin foot from Ethiopia shows multiple Pliocene bipedal adaptations’, Nature 483, 565-569

Spoor, Fred (2015). ‘Palaeoanthropology: The middle Pliocene gets crowded’. Nature 521, 432–433