Where does Ardi fit?
Ardi Compared To Gorillas, Chimpanzees and Bonobos
Ardi seems to have much more in common with the Bonobo than the Common Chimpanzee at an amateur distance.
Common Chimpanzees are 'knuckle walkers', like gorillas. Bonobos and Ardi were 'palm walkers' who use the outside edge of their palms (Lucy was a bipedal walker, much like modern humans).
Bonobos and Ardi were both probably less aggressive than Common Chimpanzees and Gorillas. Ardi did not have adaptations like sharp canine teeth associated with that kind of hierarchy and male-male competition. Neither Ardi nor Bonobos have major gender differences in size. There are no documented cases of Bonobos killing other Bonobos, while intraspecies killing is known for Common Chimpanzees, and Gorilla aggression has also been observed.
Chimpanzees also have greater size differences between males and females than either Bonobos or Ardi.
Chimpanzees: "Adults in the wild weigh between 40 and 65 kilograms (88 and 143 lb); males can measure up to 160 centimetres (63 in) and females to 130 cm (51 in)."
Ardi: "4 feet tall (120 cm) tall and weighing around 110 pounds (50 kg), Ardi was slightly shorter than Lucy but almost double her weight."
Lucy: (3 feet 8 inches) tall or a couple inches shorter and weighed 29 kilograms (65 lb).
"Height: Male: 73 - 83 cm (29 - 33"); Female: 70 - 76 cm (28 - 30").
Weight: A survey of most of the world's captive bonobos in the 1990's put the average weight of males at 43 kg (95 lb) and the average weight of females at 37 kg (82 lb)."
Gorillas, Common Chimpanzees and Bonobos all live deep in the jungle now, with both one of the Common Chimpanzee species and the Bonobos living close to Africa's Great Rift Valley. Lucy lived on the Savanna. But, this may not always have been the case. Indeed, Bonobos are sometimes called pygmie chimpanzees, and the jungle habitat that they now inhabit could reflect the same kind of dwarfism seen in the two clusters human groups of pygmies found in the Congo jungle. Ardi lived in an open, ground watered forest between the jungle and the savanna similar to forests found in parts of Kenya today, but found farther North in modern day Ethiopia at the time. Was there are Bonobo ancestor that lived in a more open forest?
Common Chimpanzees and Bonobos are specialized to eat fruit. Ardi was more omnivorous that they were, but wasn't suited to eat rougher savanna foods.
The Chimpanzee Phylogeny
The conventional wisdom based on DNA evidence is that:
[T]he Bonobo and Common Chimpanzee species effectively separated from each other less than one million years ago. The chimpanzee line split from the last common ancestor shared with humans approximately four to six million years ago. Because no species other than Homo sapiens has survived from the human line of that branching, both Pan species are the closest living relatives of humans and cladistically are equally close to humans.
It isn't clear to me if the Chimpanzee-Bonobo split is based on genetic data or the fact that chimps are found North of the Congo River, while Bonobos are found South of the Congo River and the fact that the Congo River came into being 1.5 million to 2 million years ago. The four species of Common Chimpanzee collectively outnumber Bonobos about 15-1 (there are currently about 11,000 Bonobos). The genetic data on Bonobos is apparently not very good.
The authors of the Ardi study argue that Ardi's roots are in a common ancestor of humans, the Common Chimpanzee and Bonobo, and that Common Chimpanzees and Gorillas developed their common traits independently of each other.
Few findings of the scholarly write up of Ardi have received more skepticism than the notion that the common traits of Common Chimpanzees and Gorillas do not bespeak a common ancestor with those traits. But, given the recent divergence of Common Chimpanzees and Bonobos, the alternative explanation is that Ardi and the Bonobos developed their common traits independently.
Neither is a terribly pleasing explanation.
Can I spin any better tales?
In a variation on this scenario, suppose that an Ardi was the common ancestor of humans, the Common Chimpanzee and Bonobo from the Gorilla. The DNA for Gorilla-like characteristics was turned off by not absent from Ardi's genome. Drying conditions shifted Ardi's range away from Lucy's kin. When Homo Erectus or the Neanderthals arose and were successful, Ardi was forced back from a native open forest habitat to the jungle. In one group, the Gorilla DNA lurking dormant in the Ardi genome turns back on in a jungle habitat and came to dominate the area North of the Congo River. In the other group, South of the Congo River, the Gorilla DNA stayed dormant, but jungle conditions lead to dwarfism, giving rise to the Bonobos.
In this view, Bonobos are pygmie Ardi, and Ardi existed jungle refuges until a million years ago, although not in the places where Lucy and other hominids arose.
Where Did Human Evolve?
To the extent that Ardi is a key link in the human evolutionary chain, Ardi is one of the best links for localizing geographically within Africa where human evolution took place. This is because Ardi had a quite narrow ecological niche compared to later human ancestors species. Lucy and Homo Erectus, for example, would have had a much wider range. Places within Africa that were either deep jungles or savannas in the period a little more than 4 million years ago are ruled out as locations where this significant link in human evolution could have take place. These places had to include modern Ethiopia, but could have been broader. Arguably similar skeletal remains have been found in Chad, for example.
The fact that each major genus of the hominid family tree is present in the Awash Valley of Ethiopia, which is also home to some of the earliest known modern human skeletons (about 50,000-60,000 years before humans left Africa), and the higher levels of genetic diversity found in East Africa and Ethiopia than other parts of Africa, all suggest that the Awash Valley is very close to historical Eden, and that "historical Eden" had to include the Awash Valley, even if it wasn't limited to it.
The alternate explanation is that these species of pre-humans were present in other places that were less suited to preserving skeletons or haven't been revealed by the cut of a river valley. For example, one could alternately explain the mix of genetic lineages in Africa through the expansion of the Neolithic farming Bantu people in a way that overwhelmed or replaced earlier hunter-gatherer groups. But, the best evidence that we do have points to East Africa and is largely uncontradicted, and Ardi adds an important piece of evidence to the East African Eden hypothesis.