Ardipithecus ramidus; four feet to two

In the evolution of humans, there was a point in time when our ancestors did not walk on two feet, but instead moved around on four. As the ancestors of humans evolved, they eventually reached a point where the transition to two feet began to occur. For a time, the fossils found detailing such a transition were of Australopithecus, a bipedal creature which seemed to sprout up in our evolutionary history 3.7 million years ago (1).  Australopithecus had multiple novel adaptations associated with bipedalism, causing many scientists to believe Australopithecus to be the result of a sudden evolution of bipedalism in the ancestors of humans. At the same time, other scientists believe that bipedalism was unlikely to have suddenly shown up in Australopithecus with fully formed adaptations. Instead, they believed it was the result of a more gradual process, with multiple distinct ancestors in-between. Unfortunately for scientists who believed the gradual step process, there were no fossils to support such a hypothesis until the discovery of a new species known as Ardipithecus ramidus, in November 1994, whose skeletons were discovered in Ethiopia(2).

The first skeleton found of Ardipithecus ramidus was discovered in a plain in Ethiopia known as the Afar Rift, more specifically a valley located there. This valley allows scientists to easily look for older deeper fossils unearthed there without having to dig for them, including older human ancestors who used to live there. While digging there, scientists discovered the remains of an unknown species of hominid in an area of 4.4 million year old sediment (3), but there were not enough bones to form a complete reconstruction. Subsequent analysis revealed a new skeleton dubbed ARA-VP-6/500, which was complete enough to form a reconstruction, and recreate the first Ardipithecus ramidus skeleton seen by humans. After the initial discovery, scientists continued examining the area and found multiple Ar. Ramidus fragments, which allowed them to make an analysis of the species as a whole.

The skeleton of ARA-VP-6/500 that was unearthed in Ethiopia (White et al 2009)

It is believed that the area these Ar. Ramidus lived in was a woodland biome that once existed in Ethiopia (3). There it appears that Ar. Ramidus ate an omnivorous diet, without the focus on ripe fruits seen in chimpanzees. Instead, its teeth were adapted to eating food found in both trees and on land (4). Another aspect of Ar. Ramidus’ teeth was a lack of premolar complexes, or essentially the fangs seen in gorillas and chimpanzees. This is a distinctive feature of the hominid family (the family of humans and their ancestors), and also represents a possible society where there was less competition between males. This is because upper canines are used primarily as a means of male prowess and weaponry. This supports the belief of a society with a higher level of cooperation, similar to human society seen today. (4, 5)

Another unique finding was the reduction in gender size differences in Ar. Ramidus (7), similar to what is seen in chimpanzees. This was determined by examining seven different humeral bones (upper arm bone) discovered at the site from both genders and comparing them. These smaller sexual size differences are indicative of a less competitive society, because when there is more competition among males they end up being bigger, and able to outcompete other males for food and mates. (5, 7) Therefore, this is indicative of a society with less competition, as seen in human’s society today, which the difference in teeth supports as well.

The skeletons also showed other unique features similar to humans and distinct from other species at the time. They were around 4 feet tall, with a weight that could reach 50 kilos (110 pounds) (6). Ar. Ramidus’ feet had the beginnings of many of the features of ours today as well, to allow them to walk on two legs. These included refining of the heel, and lengthening of the metatarsals, the bones in the foot preceding the toes (8). These skeletons also had hands with lengthened fingers but unchanged metacarpals, the bones before the fingers, which indicate a lack of knuckle walking (9).

In regards to their movement, they appear to have been creatures that had bipedal movement, walked upright on land, and climbed around in trees but did not have any specializations for being in trees, such as vertical climbing or suspension. They also had adaptations to the spine and the shoulders giving them greater maneuverability, and displayed a shortening of the ulna, which gave them greater forelimb maneuverability as a whole (2).

With the appearance of bipedalism, it could be interpreted that Ar. Ramidus was the move away from a mixed tree and land based environment to one that involved a larger amount of time spent on land using bipedal movement and with it the adaptation of the feet and hands to support this transition from knuckle walking which Ar. Ramidus has. Included in that is the lack of morphological specialization for being in trees that Ar. Ramidus also has, indicating the middle point in the transition to a new habitat, which is shown by the simultaneous loss of specialization for trees and beginnings of morphological specializations for land based bipedal movement. The adaptations to the spine and shoulder also show the beginnings of the extreme forearm and hand maneuverability that humans show today and allow us to use tools as well as we do, and could be a point of future research as well.

Overall, these fossils have lack of sexual size differences, dentition, hand and feet morphology, and maneuverability similar to modern humans. These factors show the beginnings of many traits seen specifically in humans today. Ar. Ramidus also shows a reasonable transition for the evolution of refined bipedalism due to spending more walking around, and which could lead the evolution of Australopithecus found 0.7 million years later, and then eventually to humans without the sudden appearance of a novel trait with many complex adaptations associated with its origination.

Citations

  1. Johanson, Donald. Taieb Maurice. Coppens, Yves. “Pliocene hominids from the Hadar formation, Ethiopia (1973–1977): Stratigraphic, chronologic, and paleoenvironmental contexts, with notes on hominid morphology and systematics” American Journal of Physical Anthropology Volume 57 Issue 4 1982. Pages 373-402.
  2. White, Dtim. Asfaw, Berhane. Beyene, Yonas. Haile-Selassie, Yohannes. Lovejoy, C. Suwa, Gen. WoldeGabriel, Giday. “Ardipithecus ramidus and the Paleobiology of Early Hominids.” Science 2 Vol 326 Number 2949 (2009) Page 75-86.
  3. WoldeGabriel, Giday. Ambrose, Stanley. Barbon, Doris…”The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus” Science 2 Vol 326 2009 page 65.
  4. Suwa, Gen. Kono, Reiko. Simpson, Scott. Asfaw, Berhane. Lovejoy, Owen. White, Tim. “Paleobiological Implications of the Ardipithecus ramidus Dentition” Science 2 Vol 326 2009 Page 69, 94-99
  5. Lovejoy, Owen “Reexamining Human Origins in Light of Ardipithecus ramidus” Science 2 Vol 326 2009 Page 74
  6. Lovejoy, Owen. Suwa, Gen. Simpson, Scott. Matternes, Jay. White, Tim. “The Great Divides: Ardipithecus ramidus Reveals the Postcrania of Our Last Common Ancestors with African Apes” Science 2 Volume 326 2009 Pages 73.
  7. Leno, Philip. Meindi, Richard. McCollum, Melanie. Lovejoy, Owen “Sexual dimorphism in Australopithecus afarensis was similar to that of modern humans” PNAS Volume 100 2003 Pages 9404-9409
  8. Lovejoy, Owen. Latimer, Bruce. Suwa, Gen. Asfaw, Berhane. White, Tim. “Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus” Science 2 Volume 326 2009 Page 72.
  9. Lovejoy, Owen. Simpson, Scott. White, Tim. Asfaw, Berhane. Suwa, Gen. “Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive” Science 2 Volume 326 2009 Page 70.