Ardi (Ardipithecus ramidus): A 4.4-Million-Year-Old Ancestor

Ardi is the nickname of a partial skeleton of Ardipithecus ramidus, a hominin that lived about 4.4 million years ago in what is now the Afar region of Ethiopia. Found in pieces from 1994 and finally published in 2009, it was then the oldest reasonably complete skeleton of a possible human ancestor. Ardi showed that early members of our lineage lived in woodland and combined tree climbing with upright walking, challenging long-held ideas about how humans came to stand on two legs.

Background: the Middle Awash

The Middle Awash is a stretch of the Awash River valley in the Afar Rift, one of the richest fossil regions in the world. Its layers of sediment and volcanic ash span more than 6 million years and have produced hominin fossils from several periods, from Ardipithecus to early Homo sapiens.

From 1981 the Middle Awash Research Project, led by the American palaeoanthropologist Tim White, the Ethiopian Berhane Asfaw and others, surveyed the area systematically. Before the 1990s, the oldest well-known hominins were the australopithecines, such as Lucy, dated to about 3.2 million years ago. Older fossils were rare and fragmentary.

Discovery at Aramis

In December 1992 the Japanese palaeoanthropologist Gen Suwa spotted a hominin molar at a locality called Aramis. Further teeth and bone fragments followed, and in September 1994 White, Suwa and Asfaw described a new species in Nature, first calling it Australopithecus ramidus. In 1995 they moved it to a new genus, Ardipithecus, because it seemed too primitive for Australopithecus.

In November 1994 the Ethiopian team member Yohannes Haile-Selassie found two bones from a hand. Excavation of the surrounding sediment revealed many more parts of the same individual, catalogued as ARA-VP-6/500. The skeleton was so fragile that bones crumbled when touched. The team removed them in blocks of sediment, hardened them with consolidant and cleaned them under microscopes in Addis Ababa.

Fifteen years of reconstruction

The skull had been crushed almost flat and the pelvis was badly distorted. Suwa used micro-CT scans to rebuild the skull digitally from dozens of fragments, while the American anatomist C. Owen Lovejoy led work on the pelvis and limbs. Altogether, Aramis has produced more than 100 Ardipithecus ramidus specimens representing at least 36 individuals, which helped the team interpret Ardi.

The results appeared on 2 October 2009 in a special issue of Science containing 11 papers by an international team of 47 researchers. The journal later named Ardi its scientific breakthrough of the year.

What Ardi was like

Ardi was probably a female, standing about 120 centimetres (just under 4 feet) tall and weighing roughly 50 kilograms. Her brain was small, at around 300 to 350 cubic centimetres, similar to a chimpanzee’s.

Her body was a surprising mosaic:

• Feet: The big toe stuck out sideways and could grasp branches like a thumb, but the outer part of the foot was stiffened, which the team argued helped push off the ground when walking upright.
• Pelvis: The upper blades of the pelvis were short and broad, like those of later hominins, suggesting upright walking. The lower part remained ape-like, anchoring powerful climbing muscles.
• Hands and arms: Long arms and flexible hands suited careful climbing along branches. Ardi lacked the stiff wrists of chimpanzees and gorillas, which walk on their knuckles.
• Teeth: The canine teeth of both sexes were small and blunt, without the large sharpened canines of male apes.

From this, the team concluded that Ardi moved carefully on all fours in trees and walked upright on the ground, though less efficiently than later hominins.

A woodland world

The animal and plant fossils from Aramis include woodland species such as colobus monkeys, kudu, many kinds of birds and small mammals, as well as fossil seeds and wood. The researchers argued that Ardi lived in a fairly closed woodland, not on open grassland.

This mattered because a popular idea, the savanna hypothesis, held that our ancestors began walking upright after leaving forests for open plains. If Ardi walked upright in woodland, that idea needed revision. In 2010 Thure Cerling and colleagues challenged the habitat reconstruction, arguing that the soil chemistry pointed to a more open, tree-or-bush savanna. The disagreement has not been fully resolved.

Debate and significance

The most far-reaching claim was that Ardi showed the last common ancestor of chimpanzees and humans was not chimp-like. White and Lovejoy argued that knuckle-walking and other specialised features of modern African apes evolved separately after the split. Lovejoy also linked the small male canines to reduced competition between males and to pair bonding, a hypothesis many scientists regard as speculative.

Some researchers questioned whether Ardi was a hominin at all, suggesting that the evidence for upright walking depended heavily on the reconstruction of a crushed pelvis. Others accept Ardipithecus as an early hominin but debate whether it was a direct ancestor of Australopithecus or a side branch. Most specialists agree that Ardi provides the best view yet of hominin anatomy before about 4 million years ago.

Ardipithecus and its relatives

An older species, Ardipithecus kadabba, is known from teeth and bones found in the Middle Awash and dated to about 5.8 to 5.2 million years ago. Even older candidates for the earliest hominin include Orrorin tugenensis from Kenya, at about 6 million years, and Sahelanthropus tchadensis from Chad, at about 7 million years. The first Australopithecus, A. anamensis, appears around 4.2 million years ago, not long after Ardi.

Myths and facts

Myth: Ardi proves that humans did not evolve from apes. When Ardi was announced, some headlines suggested this. In fact, no scientist claims that humans descend from modern chimpanzees; both lineages descend from a shared ancestor that was itself an ape. What the Ardi team argued was that this common ancestor did not look much like a living chimpanzee.

Myth: Ardi is “the missing link”. The phrase implies a single fossil connecting apes and humans. Human evolution is better pictured as a branching bush with many species, and Ardi is one important branch point in that picture rather than a single link in a chain.

Fact: Ardi was not found complete. More than 125 pieces of the skeleton were recovered, including much of the skull, teeth, pelvis, hands and feet, but many parts are missing or badly damaged, and some of the reconstruction relies on careful interpretation.

By the numbers

Timeline

• 1981: The Middle Awash Research Project begins fieldwork in the Afar region.
• 1992, December: Gen Suwa finds the first Ardipithecus ramidus tooth at Aramis.
• 1994, September: The species is named Australopithecus ramidus in Nature.
• 1994, November: Yohannes Haile-Selassie finds hand bones belonging to the Ardi skeleton.
• 1995: The species is moved to the new genus Ardipithecus.
• 2009, 2 October: Science publishes 11 papers describing Ardi and her environment.
• 2009, December: Science names Ardi its breakthrough of the year.

Why it matters

Ardi gave scientists their first detailed look at a hominin from more than a million years before Lucy. Her blend of climbing and walking adaptations showed that upright walking began in a creature still at home in the trees, and that the path from ape-like ancestor to human was not a simple straight line. Later finds, such as the Laetoli footprints, show how far bipedalism had developed within the following million years.