The science of episode one: First Ancestors
Why did we choose to begin our story with afarensis?
Our story begins around 3.5 million years ago in Ethiopia. As one of the first species of upright walking bipedal apes, we featured Australopithecus afarensis in our opening story because this is certainly the best-documented early hominid. (‘Hominid’ is the name we give to the upright walking ape-like ancestors of humans.)
The female afarensis known by scientists as Lucy (find AL288-1) has now become a celebrity fossil. It contains 47 out of 206 bones in a full skeleton, which might not sound that impressive, but in the world of palaeoanthropology it is outstanding.
We can deduce a fair amount about the group make-up of afarensis from a famous fossil discovery AL333, known as The First Family. Found by Michael Bush in 1975, this discovery comprises the remains of at least 13 individuals that were buried in a catastrophic event, possibly a flood. The group appears to have been related to each other. This collection provides unique insights into the structure and biology of this species.
How did we model afarensis' behaviour?
The fossil evidence gives us some clues about afarensis' social life, but to help build up a more complete outline we have tried to model their behaviour on that of chimpanzees, because they are closely related to early hominids.
Jane Goodall's work at Gombe, Tanzania, is invaluable in outlining the complex political world of these extraordinary social animals. She has documented chimpanzees' complicated hierarchies of dominance, display behaviour and male/female relationships.
Our key storyline, the territorial disputes between Lucy's group and a neighbouring troop, is based largely on actual chimpanzee observations. Goodall writes of one incident where a group of patrolling chimps encounter a female with an infant. One of the males seized the stranger, hit and bit her, before stamping on her back. During the fierce assault, another male seized the infant and charged off through the bushes. In this way the males of a troop protect food resources for their own females and young.
What do we know about the common ancestor?
Genetic evidence suggests that chimps and upright walking ape ancestors diverged from a common ancestor between 5 and 7 million years ago. Fossils of this mysterious creature have never been found, as the dense forest environments that would have been its home do not preserve fossil remains well. Like living apes that are adapted to living in the forest, the common ancestor probably walked on its knuckles.
The changing climate and environment
Many experts believe that the key to the creation of the broken, mixed environment of trees, shrubs and savannah grasslands in Africa in which afarensis evolved, is the monsoon. Many millions of years ago, India collided with the continent of Asia, buckling the surface of the Earth to create the huge mountain range that we call the Himalayas.
This geological event created a monsoon that released vast quantities of rain, drying out the air. This same air flows across East Africa, and causes rainfall to drop sharply, drying the rain forests and replacing them with broken scrub and woodland. It is no coincidence that the monsoon intensified 6-8 million years ago - the time at which the common ancestor lived.
So why did bipedal apes emerge in this environment?
Paleoanthropologists have advanced many theories over the years as to why quadrapedal (four-legged) apes began walking upright. One of the most recent, and persuasive, theories has been suggested by Dr Patricia Kramer of the University of Washington, who believes that bipedalism was the most energy-efficient way of moving around the broken landscape that appeared in Africa between 6 and 8 million years ago.
Energy is simply too critical a commodity to waste if you are a primate living a marginal existence in the African savannah. Its conservation becomes critical. Dr Kramer found that the short-legged morphology of early bipeds was the most energy-efficient body shape for covering relatively long daily distances on the ground. Once bipedalism took hold, there was no looking back.
Next: Episode two - Blood Brothers