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Lec. 10 Pre-Australopithecines & Australopithecines
Transcript of Lec. 10 Pre-Australopithecines & Australopithecines
Gradistic & Cladistic
Some people say hominid and others say hominin. This is because of that transition from gradistic to cladistic.
Before carrying on with human ancestors, it is necessary to make sure you are familiar with the proper terminology.
Remember that when organic remains are rapidly buried in an oxygen-free environment with time and pressure, it becomes a fossil. Oxygen free environments include rock, tar, ice, amber, and bogs!
Australopithecines were a very successful group of hominins that lived from 4.2-1.4mya in South and East Africa. All of them were capable of walking bipedally, though not efficiently, all had small brains, large faces and large teeth.
Hylobatidae Pongidae Hominidae
Gibbon Orangutan Humans &Siamangs Gorillas, Chimps & ancestors
In gradistic classification, none of the other apes are closely related to humans or human ancestors.
Gibbons & Siamangs
Humans, chimps, bonobos, & human ancestors
Notice that in the cladistic, apes are more closely related to humans.
There are two types of fossil.
Body: fossils that reveal the body structure of organisms.
Trace: reveals the behavior of an organism. These would be things like footprints or tools.
Before the early 20th century, most paleoanthropology was focused on comparisons with non-human primates.
It wasn't until Eugene Dubois found a fossil on the island of Java that the field changed.
Dubois started comparing fossils to oeach other and to modern humans and revolutionized the study of evolution.
While Dubois' find wasn't the first hominin fossil, it was the first to purposely search for fossils for the purpose of comparing it to others.
Dubois' path to prominence was an interesting one. He started as a lecturer at University of Amsterdam. He quit that job to become a physician in the Dutch colonial government in the Dutch East Indies (Indonesia). He then quit that job and became a fossil hunter in Java and Sumatra. Talk about your mid-life crises.
Great Rift Valley
The majority of human evolution fossils come from East or South Africa. Particularly the area of the Great Rift Valley in East Africa.
Because of plate techtonics, old geological beds are exposed and come to the surface. In the same region, volcanic sediment used to be common in the area, perfect for making fossils.
These fossil hominins are in chronological order. These are all we have to this point, but surely there will be more, just look how recent some of these finds are.
Please be aware, I will not ask you to know dates, but I will ask that you know the species of specific fossil finds.
Found in Sahel, Chad and dates to around 7-6mya, just after the time we believe humans and chimps split (8-10mya).
The skull has small teeth with thick enamel and strong brow ridges.
What is really interesting, though, is the very flat face, not seen again until genus Homo.
The brain case is very small, only about 350 cubic centimeters (cc). This is actually smaller than modern chimpanzees.
Thus, Sahelanthropus tchadensis has an interesting combination of ancestral and derived traits.
Some argue it is not a hominin, but based on the foramen magnum we consider it possibly bipedal and therefore a hominin.
Found in 2002
Dating from 6-5.8mya, Orrorin was found in the Tugen Hills in northern Kenya.
There is not much comprising this fossil, mainly a couple of long bones and dental remains. Despite all the missing pieces, Orrorin is believed to be a biped because of the long femoral neck similar to modern human femurs.
The pointed canines are very ape-like, which means we have a blend of ancestral and derived features.
Found in 2001
& Ardipithecus kadabba
Also known as 'Ardi', Ardipithecus lived between 5.8-4mya around Ethiopia. Tim White was one of the foremost researchers of Ardipithecus, and it took scientists like him 15 years to clean and study the bones.
Ardi is the most complete pre-australopithecine hominin we have and has an odd mixture of ancestral and derived features.
Small brain case,
Long, curved arms & fingers (for living in trees)
Central foramen magnum.
Short & broad iliums,
The reduced canines suggest Ardi might have been living in co-operative societies with less need for strict dominance hierarchy.
The combination of arboreal (ancestral) and bipedal (derived) features suggests that Ardi was living in a patchy forest habitat and needed to exploit living in trees and on the ground.
Notice that the three pre-australopithecines are not directly connected to any of the other hominin ancestors. That is because we are not sure if or how they fit into our lineage. We believe they are part of our ancestry because they are likely bipedal, but for all we know they are evolutionary dead ends that didn't lead anywhere and our earliest ancestor has yet to be found.
There is another group called the Paranthropines that some consider to be robust Australopithecines while others think they are a completely different genus from Australopithecus called Paranthropus.
For good measure, there is one other misfit species occurring around the same time.
Looking at this phylogeny again, we can see Kenyanthropus platyops has unknown origins, but might connect to a member of the genus Homo. The Paranthropus genus appears to be an evolutionary deadend. Currently the most likely ancestral species to our genus is either Australopithecus ghari or Australopithecus sediba. However, there is still the possibility we have not found the true common ancestor.
There are also some evolutionary patterns that emerge. For instance, these ancestors had a limited geographic range (E and S Africa). They were all still tied to the trees to an extent (not obligate bipeds). The brain size did not increase greatly at all. And we believe (based on fossil infants) they had an slower period of growth and development similar to humans rather than apes.
This is particularly important because a slow growth rate is necessary because as it slows, humans will be able to keep the small brain size at time of birth, but develop the larger brain during childhood.
This fossil was discovered by Meave Leakey in 1994. The Leakey family is heavily involved in paleoanthropology and you will see them again. Au. anamensis lived between 4.2-3.9mya.
Like all of the Australopithecines, Au. anamensis has both ancestral and derived traits. It has long robust roots for its canine teeth, like an ape. It has thick enamel and large molars. But it also has bipedal features, such as the proximal and distal end of its tibia.
The large molars are a sign of ape like dentition. Larger molars mean the dental arcade is more parallel. Notice that Au. anamensis has parallel dental arcade. However, you can see the front teeth are starting to take on a parabolic shape.
Au. afarensis is perhaps the most famous species of hominins because of the variety of finds.
Au. afarensis lived from 3.7-3.0mya in East Africa. It had very ape-like features such as a small brain case and prognathic face. However, Au. afarensis also had very definitive bipedal features. For instance, they have a bipedal knee joint and broad os coxae. There are several fossils of Au. afarensis that you should be familiar with, most importantly...
Easily the most famous hominin is Lucy. This fossil is 3.2mya, 40% complete. She is an adult female (known because of her erupted 3rd molar), about 4ft tall and around 12yrs old.
She was found by Donald Johanson and was named because the Beatles' song 'Lucy in the sky with diamonds' was playing during their celebrations.
The Laetoli footprints were found in Laetoli, Tanzania by Mary Leakey. These are the most famous trace fossil. Footprints were made in volcanic ash that show clear bipedal locomotion. There is no divergent toe which is similar to humans. There are at least 2 individuals, but possibly as many as 4. Looking carefully we can see heels inside one set of prints, and some have suggested the heels of the smaller prints are really deep meaning it was a pregnant female.
Also around 3.2mya lived the first family, a group of 7 individuals (5 adults and 2 children). Comprised of 200 skeletal fragments, this find confirmed multiple things. Firstly, we now know these individuals lived in larger groups. We also know Au. afarensis is sexually dimorphic, with males taller and heavier than females.
It is assumed these individuals lived near a body of water, were overcome by a flood, and preserved after rapid burial.
Selam was discovered in 2000, but it took 8 years to remove all the sand particles from the fossil. It is the oldest child fossil yet discovered. Selam was only 3yrs old when she died, we know that from her dental eruption. Perhaps most interesting, the spinal column and scapula are tucked under the skull. We can tell it was partially bipedal from its foramen magnum, but we can also tell it was arboreal based on its scapula.
Scapula research VERY recent!
Another thing we have learned from selam is how quickly Australopithecines develop. While the brain itself cannot fossilize, the shape did. We can see from the endocast that the brain is 75% of its adult size at the age of 3. This means the growth is slower than a chimpanzee.
Therefore, we know hominin development has slowed down and is starting to resemble human growth and development. The building blocks for a large brain size are in place.
Au. africanus (not to be confused with Au. afarensis) lived between 3.3-2.5mya. The prognathism has decreased and it is a more gracile species than Au. afarensis, including having smaller teeth (though still larger than ours).
Au. africanus is a habitual biped with a human-like pelvis and a more human-like curve to their spine. However, it still does not have a striding gate like the obligate bipeds in the genus Homo. Also, the fingers are not curved which suggests it is starting to lose the arboreal features of its ancestors.
Raymond Dart was credited with the discovery of Taung Child. This was the first hominin fossil found in Africa and ultimately debunked Piltdown Man. Taung Child might have been killed by an eagle or bird of prey, a conclusion reached because of damage to the skull and eye sockets similar to how other primates are killed by raptors. This is another instance of the endocast preserving to give us some knowledge about their brain.
Au. garhi lived roughly 2.5mya and is largely credited to Tim White (from Ardipithecus)and Berhane Asfaw. While it has a small brain, it continues the trend of becoming more gracile. The teeth are larger than ours, but not specialized at all, very generalized dentition. It has been suggested that Au. garhi might have made stone tools, though this has not been confirmed. It is also possible this species is the ancestral link to the genus Homo.
Au. sediba was discovered in 2009 and announced in 2010. It lived 1.94-1.78mya and is currently being speculated as the current missing link between Australopithecus and Homo. Only 2 individuals have been uncovered, a juvenile male and adult female, but at least 7 more are yet to be uncovered in the same location. Au. sediba shows continued mixture of ancestral and derived traits, but appears to be a more energy efficient walker/runner very similar to Homo species.
Au. sediba still has a small brain (420-450cc) that is only about 1/3 the size of H. sapiens.
It also still has long arms which suggests at least a partial arboreal lifestyle.
Au. sediba has smaller teeth. It also has short fingers and a long thumb allowing for better precision grip and more manual dexterity.
Au. sediba's pelvis is human-like, it has longer legs and features indicate it is more consistently bipedal (though still not obligate).
Genus Paranthropus (contemporaneous with Australopithecus)
The paranthropines are very robust and very prognathic. They have large cheek teeth (molars) that are 4x the size of humans. They also have smaller incisors which suggests they do not use them for consuming foods. Finally, they all have a sagittal crest which indicates they were chewing hard foods. We believe they are evolutionary dead ends because they are so extremely large and most extreme forms die out when environments change.
This individual comes from East Africa (like Ethiopia) and lived about 2.5mya. The most famous of these is called the 'black skull' because the fossil was died black by the sediment it was found in. These features are both ancestral and derived, which makes sense because it is the earliest paranthropine known and probably derived from one of the australopithecines.
Note, the sagittal crest is considered a derived trait because it was lost with the Australopithecines and Ardipithecus, but reoccurs with the Paranthropines.
P. robustus is the only paranthropine found in Southern Africa and lived between 2-1.4mya. This means it COULD BE (not guarenteed either way) completely unrelated to the other paranthropines. Interestingly it has a much smaller sagittal crest than the other two paranthropines, but is called robustus. That is because it was the first one found. Scientists figured it was so huge it should be called robustus, but them found out it wasn't as big as others. Unlike P. aethiopicus and P. boisei (both of which live in East Africa and feed on mainly plant material), P. robustus has a mixed diet with meats and plants.
P. robustus is the main reason there is debate over whether Paranthropus is a separate genus or just a robust group of Australopithecines. It has a similar diet to Au. africanus and is not very similar to P. aethiopicus or P. boisei (except that it is large).
P. boisei lived 2.4-1.4mya in East Africa. It was found by Mary and Louis Leakey and originally called Zinjanthropus boisei (ZINJ). This is the most robust form of the paranthropines. It mostly ate grasses, unlike P. robustus. It is likely P. boisei is a derived version of P. aethiopicus.
Known as the flat-faced man from Kenya, Kenyanthropus lived 3.5-3.2mya (contemporaneous with the Australopithecines and Paranthropines). It has uncertain ancestral origins, but resembles some species of the genus Homo. It's possibly the ancestor to Homo, but Australopithecines are thought to be the more likely candidate.
Despite the obvious skull size difference, there is no difference in body size between Paranthropus and Australopithecus. These groups were sympatric, living in the same location but feeding on different resources (except P. robustus which ate the same foods as the Australopthecines). However, Paranthropines are more robust, bowl-shaped face, large cheek teeth, and sagittal crests. Australopithecines are more gracile, prognathic, have smaller teeth and no sagittal crest. Ultimately it is your choice whether you consider them separate genera or members of the same genus, Australopithecus.
Finally, think how interesting it is that there were so many branches to our ancestral history. Think of what it took for our genus, Homo, to have emerged from this jumble of bipeds when no other groups survived. And imagine walking around and seeing other bipedal creatures, that aren't quite human, but aren't quite ape either. We are a very unique species, but our ancestry is very diverse.