Human Ancestors - Ardipithecus Group

Evolution
Colin Keates / Getty Images

The most controversial topic within Charles Darwin's Theory of Evolution through Natural Selection revolves around the idea that humans evolved from primates. Many people and religious groups deny that humans are in any way related to primates and instead were created by a higher power. However, scientists have found evidence that humans did indeed branch off from primates on the tree of life.

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The Ardipithecus Group of Human Ancestors

Ardipithecus ramidus specimen
By T. Michael Keesey (Zanclean skull Uploaded by FunkMonk) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons​

The group of human ancestors that are most closely related to the primates are called the Ardipithecus group. These earliest humans have many characteristics that are similar to apes, but also unique traits that resemble those of humans more closely.

Explore some of the earliest human ancestors and see how the evolution of humans all started by reading the information of some species below.

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Ardipithecus kaddaba

Hadar, Ethiopia
Australopithecus afarensis 1974 discovery map, Creative Commons Attribution-Share Alike 3.0 Unported license

Ardipithecus kaddaba was first discovered in Ethiopia in 1997. A lower jaw bone was found that did not belong to any other species that was already known. Soon, paleoanthropologists found several other fossils from five distinct individuals of the same species. By examining parts of arm bones, hand and foot bones, a clavicle, and a toe bone, it was determined this newly discovered species walked upright on two legs.

The fossils were dated to be 5.8 to 5.6 million years old. A few years later in 2002, several teeth were also discovered in the area. These teeth that processed more fibrous foods than known species proved this was a new species and not another species found within the Ardipithecus group or a primate like a chimpanzee because of its canine teeth. It was then that the species was named Ardipithecus kaddaba, which means "oldest ancestor".

The Ardipithecus kaddaba was about the size and weight of a chimpanzee. They lived in a wooded area with a lot of grass and freshwater nearby. This human ancestor is thought to have survived mostly off of nuts as opposed to fruit. The teeth that have been discovered show that the broad back teeth were the site of most chewing, while its front teeth were very narrow. This was a different dental set up than primates or even later human ancestors.

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Ardipithecus ramidus

Ardipithecus skull
By Conty (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons

Ardipithecus ramidus, or Ardi for short, was first discovered in 1994. In 2009, scientists unveiled a partial skeleton rebuilt from fossils found in Ethiopia that dated to about 4.4 million years ago. This skeleton included a pelvis that was designed for both tree climbing and walking upright. The foot of the skeleton was mostly straight and rigid, but it had a large toe that stuck out the side, much like a human's opposable thumb. Scientists believe this helped Ardi travel through the trees when searching for food or escaping from predators.

Male and female Ardipithecus ramidus was thought to be very similar in size. Based on Ardi's partial skeleton, the females of the species were about four feet tall and somewhere around 110 pounds. Ardi was a female, but since many teeth have been found from several individuals, it seems that males were not much different in size based on canine length.

Those teeth that were found give evidence that the Ardipithecus ramidus was most likely an omnivore that ate a variety of foods including fruit, leaves, and meat. Unlike theArdipithecus kaddaba, they are not thought to have eaten nuts very often since their teeth were not designed for that sort of tough diet.

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Orrorin tugenensis

O. tugenesis is known as "Millenium Man"
Lucius/Wikimedia Commons

Orrorin tugenesis sometimes called "Millenium Man", is considered part of the Ardipithecus group, even though it belongs to another genus. It was placed in the Ardipithecus group because the fossils that were found dated back from 6.2 million years ago to about 5.8 million years ago when the Ardipithecus kaddabawere thought to have lived.

The Orrorin tugenensis fossils were found in 2001 in central Kenya. It was about the size of a chimpanzee, but its small teeth were similar to that of a modern human with very thick enamel. It also differed from the primates in that it had a large femur that showed signs of walking upright on two feet but also were used for climbing trees.

Based on the shape and wear of the teeth that have been found, it is thought that theOrrorin tugenensis lived in a wooded area where they ate a mostly herbivorous diet of leaves, roots, nuts, fruit, and the occasional insect. Even though this species seems to be more ape-like than human, it did have the hallmarks that lead to the evolution of humans and could be the first step from primates evolving into modern day humans.

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Sahelanthropus tchadensis

Cast of the Sahelanthropus tchadensis holotype cranium
By Didier Descouens (Own work) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons​

The earliest known possible human ancestor is the Sahelanthropus tchadensis. Discovered in 2001, a skull of the Sahelanthropus tchadensis was dated to have lived between 7 million and 6 million years ago in Chad in Western Africa. So far, only that skull has been recovered for this species, so not much is known.

Based on the one skull that has been found, it was determined that the Sahelanthropus tchadensis walked upright on two legs. The position of the foramen magnum (the hole through which the spinal cord comes out of the skull) is more similar to a human and other bipedal animals than an ape. The teeth in the skull were also more like a human's, especially the canine teeth. The rest of the skull features were very ape-like with the sloping forehead and small brain cavity.

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Scoville, Heather. "Human Ancestors - Ardipithecus Group." ThoughtCo, Jan. 4, 2018, thoughtco.com/human-ancestors-ardipithecus-group-1224794. Scoville, Heather. (2018, January 4). Human Ancestors - Ardipithecus Group. Retrieved from https://www.thoughtco.com/human-ancestors-ardipithecus-group-1224794 Scoville, Heather. "Human Ancestors - Ardipithecus Group." ThoughtCo. https://www.thoughtco.com/human-ancestors-ardipithecus-group-1224794 (accessed January 23, 2018).