Meet Ardi - a mother to all of humanity

 

She was 1.2 metres tall, weighed 50 kg, lived in Ethiopia about 4.4 million years ago, and she tells us some surprising things about the path of human evolution, writes DICK AHLSTROMScience Editor

A DETAILED ANALYSIS of human-like fossils discovered in Ethiopia has taken mankind back in time, a million years closer to the “missing link” ancestor we share with the great apes. In the process, scientists have been forced to rethink human evolution and what the missing link must have looked like.

It overturns the widely held view that chimpanzees and gorillas must be typical of the look and behaviour of our shared ancestors. The research proves that the modern apes are very poor models for the last common ancestor, opening up a whole new perspective on the separate tracks taken by ape and human evolution. A staggering amount of data has come from an intensive study, lasting several years, of a human-like or hominid animal called Ardipithecus ramidus.

Scientists have reduced this to “Ardi”, to describe a female A. ramidus who last walked on Earth about 4.4 million years ago in what is now the Afar Rift in the Middle Awash river valley of northeastern Ethiopia.

Fossilised skeletal remains of this animal were first found and named in the early 1990s by Prof Tim White of the University of California, Berkeley. More than 100 fossil pieces were discovered over time, including teeth and bones from at least 36 individuals, but there among the scraps was an amazingly complete skeleton, Ardi. She was about 45 per cent intact, and the fragments included her skull, with face and jaw pieces, her arms and her hands.

Discussion and controversy immediately followed as scientists debated what exactly had been found. Some rejected the notion that this was a hominid ancestor, but White and his colleagues named the genus Ardipithecus as more fossils were found.

What was not in dispute was the age of the fossils which, because of their location, sandwiched between lava flows, could be dated to about 4.4 million years ago. This is 1.2 million years older than “Lucy”, the 3.2 million-year-old-fossil remains of a different animal, Australopithecus afarensis, who without dispute forms a link in the human evolutionary chain.

This morning, the first major analysis of Ardi is presented in the journal Science, conclusively adding her to our hominid evolutionary lineage. The journal presents the work of 47 scientific authors from around the world led by Prof White and collaborators including Dr Berhane Asfaw of the Rift Valley Research Service in Addis Ababa.

The scientists undertook an exhaustive study of Ardi, but also searched the fossil record to learn what her environment was like, what plants grew there when she lived and what animals and insects shared the space with her.

It has allowed them to describe with complete confidence how she lived and what she ate. Her fossilised bones have also filled in huge gaps in our knowledge of hominid evolution, even back to the famous missing link. The authors described her as having as a mix of “primitive” traits, those shared with the animals that came before Ardi, but also “derived” traits, which she shared exclusively with later hominids.

Ardi stood about 1.2m tall and weighed about 50kg. The authors conclude that Ardi represented a surprising mix, not just of ape-like features, but startlingly human traits such as upright walking, wrist and hand bones very different from those of modern apes, and human-like teeth.

She could walk without the exaggerated swinging gait seen in modern apes. The bones in her feet were stiffer to help her walk and probably to run, yet her feet retained the ape’s grasping big toe. “It is so rife with anatomical surprises that no one could have imagined it without direct fossil evidence,” the authors write.

Her pelvis shows she was no “knuckle-walker” but strode forward with her hands free. While she had the grasping hands and feet needed to scale trees easily, she was not adapted for swinging and hanging from tree branches like today’s apes and monkeys.

For this reason, they describe her as a “careful climber”, as comfortable in the trees as on the ground. The authors also suggest that her hand structure meant she was already relatively dextrous and better able to manipulate objects than modern apes.

The human-like canine teeth were particularly surprising. These animals did not have the vicious dagger-like canine teeth seen in modern chimps and gorillas, suggesting they lived a very different lifestyle than today’s apes.

The big canines in modern apes are specially designed to rub against lower teeth and remain as sharp as possible. Chimps use them in support of reproductive success, threatening or harming rivals to gain access to fertile females. Yet this early hominid has canines much like those of modern humans.

The authors believe the small canines point towards a completely different reproductive approach that “transformed the social structure of early hominids”. Male-to-male aggression was obviously not necessary for reproductive success.

“That shift probably reduced male-to-male conflict and combined three previously unseen behaviours associated with their ability to exploit both trees and the land surface,” the authors write.

One behaviour was regular food-carrying, foraging for food items in a variety of areas such as ground, trees and shoreline, and then returning with these to the troop. Detailed chemical analysis of Ardi’s teeth showed she and her fellows were omnivores, eating a wide range of foods.

The next two behaviours are linked. They are pair-bonding and reproductive “crypsis”, where the females do not advertise ovulation and fertility, as is the case with chimps. Greater reproductive success would have come from an “intensified male parental investment” if the male was unable to recognise ovulation. The authors describe this as “a breakthrough adaptation with anatomical, behavioural, and physiological consequences for early hominids and for all of their descendants, including ourselves”.

Chimps, bonobos and gorillas are undoubtedly our closest living ancestors, but how does this now match up in light of the findings related to Ardi? Scientists believe that the missing link, our last shared common ancestor with the apes, lived at least six million years ago. Ardi undoubtedly exhibits traits that would be seen in this earlier ancestor if its fossils could be found.

Ancient ape descendants derived from the common ancestor would presumably also have some of these characteristics. The only conclusion when looking at today’s apes is that their lineage underwent their own extensive adaptation and evolution over time to deliver the apes we see today.

For this reason, the authors stress that it is no longer possible to use chimps and modern great apes as a “proxy” for the missing link.