Paranthropus robustus

Paranthropus robustus is a species of robust australopithecine that lived in South Africa between about 2.27 and 0.87 million years ago. Fossils of this ancient human relative have been found in several caves in the Cradle of Humankind, including Kromdraai, Swartkrans, Sterkfontein, and Gondolin. Discovered in 1938, it was one of the first early hominins to be described and helped scientists learn more about our ancestors.

This species is known for its very strong skull and powerful jaws, which allowed it to chew tough foods. Males were larger than females, and the species may have had a mix of tree climbing and walking on two legs. P. robustus likely ate plants found in open grasslands, as well as fruits, roots, and maybe even honey and termites. Evidence suggests they may have used bones as tools to help prepare their food. They lived in areas with mixed open and wooded landscapes and faced predators like sabertooth cats, leopards, and hyenas. Their presence in caves might be due to these big cats bringing their bodies there.

Taxonomy

Research history

Discovery

In 1938, a schoolboy in South Africa found the first pieces of a very old human-like being. He gave them to a scientist who then passed them to another scientist, Robert Broom. Broom started looking at the spot where they were found and soon found more pieces, including parts of arm bones and a toe bone. He noticed these pieces were much stronger than other early human relatives he had seen. Broom decided to call this new group Paranthropus robustus. The name Paranthropus means “beside humans” in old Greek.

At the same time, another scientist, Raymond Dart, had found an even earlier human relative called Australopithecus africanus in South Africa. Broom also found other similar fossils in nearby caves. He thought these showed that many different kinds of early human relatives lived together a long time ago.

"P. crassidens"

Later in 1948, Broom found more fossils in a different cave close by. He thought these were a different kind of early human relative and called them P. crassidens. Scientists argued about whether these were really different or just variations of the same group. Over time, most scientists decided that P. crassidens was probably the same as P. robustus.

Gigantopithecus

Broom also thought P. robustus might be related to a huge, ancient ape called Gigantopithecus found in Asia. But later studies showed Gigantopithecus was actually a type of orangutan, not a human relative.

P. boisei

In 1959, scientists found an even stronger type of early human relative in East Africa, called P. boisei. Later discoveries showed that P. boisei lived at the same time as an early human ancestor called Homo erectus. Scientists still discuss how these groups are related and which came first.

Classification

The group Paranthropus now includes P. robustus, P. boisei, and P. aethiopicus. Scientists are not sure if these all belong together in one group or if they should be separated. Because we have only found a few bones, it is hard to know exactly how they are related. Some scientists think P. aethiopicus was the ancestor of P. boisei, while P. robustus came from an earlier branch. In 2023, scientists found very old genetic material from teeth of P. robustus, the oldest human genetic material ever found.

Anatomy

Head

Skull

P. robustus had very large cheek teeth but smaller front teeth, similar to other Paranthropus species. The premolars looked like molars, and the enamel on the cheek teeth was thick, like in modern humans. Males had a tall face with a bony ridge on the top of the skull and big cheekbones, which helped with strong biting. The cheeks stuck out so much that the nose looked like it was in a dip when viewed from above. This changed the position of the eye sockets and created a weaker brow ridge. The strong chewing muscles also pushed the teeth rows back, creating a stronger bite on the premolars. The lower jaw was tall, which helped with powerful biting.

The female skull DNH-7 did not have the same strong features as the males, suggesting males were more heavily built. The Drimolen fossils were more lightly built and probably had a weaker bite than the fossils from Swartkrans and Kromdraii. Their brows were rounded, and the skull ridge was positioned more towards the back of the head.

The inner ear of P. robustus was different from that of apes or humans, suggesting different ways of moving and balancing. This might mean P. robustus was not good at long-distance running.

Brain

Early estimates of the brain size of P. robustus varied a lot. Some early guesses were around 600 cc, but later studies suggested it might have been smaller. In 2020, a nearly complete skull was measured to have a brain volume of about 450 cc.

Blood vessels

Studies of the blood vessels in P. robustus skulls showed some differences from earlier hominins and modern humans. These changes might be related to how blood flow was organized as these early humans started walking on two legs.

Torso

Few spine bones are known for P. robustus. The known spine bones show some similarities to both humans and other apes. The pelvis was similar to that of other early hominins but had some features that suggest it was used for walking on two legs, though it might have worked a bit differently than in modern humans.

Limbs

The arm bones of P. robustus were similar in size to both humans and chimpanzees. The wrist could move like a human’s but had some features that might suggest some climbing. The hand bones were like humans’, with strong muscles for gripping, which might have helped with making or using simple tools.

The leg bones suggest P. robustus walked in a way that was a bit different from modern humans, possibly less efficient. The hip joint showed signs of strong walking but also some climbing. The knee and foot bones show a mix of features from humans and apes, suggesting P. robustus could walk and possibly climb trees.

Size

Early guesses about the size of P. robustus varied widely. Some early estimates suggested they were quite heavy, around 68–91 kg (150–201 lb), but later studies suggested they were lighter, around 30–43 kg (66–95 lb), with males generally larger than females. These estimates help scientists understand how P. robustus might have looked and moved around.

Palaeobiology

Diet

In 1954, Robinson suggested that the strong build of P. robustus and its powerful bite were adaptations for eating hard foods like nuts. Later studies looked at the muscles of the face, wear on teeth, and comparisons with other animals. In 1981, Alan Walker studied P. boisei skulls and suggested that the strong build allowed these ancient humans to chew large amounts of food at once. He also found that wear on teeth matched patterns seen in modern monkeys and apes.

In 2004, Bernard Wood and David Strait concluded that Paranthropus were general eaters, and that P. robustus ate a mix of foods. They found that wear on teeth suggested hard foods were not eaten often, and the strong skull was useful when eating less preferred foods. This is similar to modern gorillas, which can survive on lower-quality foods year-round. In 2005, Greg Laden and Richard Wrangham proposed that Paranthropus may have relied on underground plants like roots and tubers, which could explain their strong teeth.

A 2006 study using carbon isotopes suggested that P. robustus ate plants from both savannas and forests, depending on the season. They also likely cracked hard foods like seeds or nuts sometimes. A high rate of cavities could mean they ate honey. Young P. robustus may have eaten more tubers than adults. Wear on baby teeth could mean they ate more gritty foods or had trouble cleaning food from their teeth.

Social structure

Given the big differences in body size between males and females, some scientists think P. robustus may have lived in groups where males were larger and more dominant. In 2007, Charles Lockwood and colleagues suggested that the size difference between males and females was similar to that of modern gorillas, where one male leads a group of females. However, studies in 2011 and 2016 suggested that females may have left their birth groups more often than males, which makes a gorilla-like society less likely. If P. robustus lived in open areas, having multiple males in a group may have helped protect them from predators, similar to modern baboons.

In 2017, Katharine Balolia and colleagues suggested that the large crests on male skulls might have been used for showing off, similar to how male gorillas and orangutans use their features to attract mates or show strength. This is different from other animals that show off by flashing their teeth, which Paranthropus likely did not do because their teeth were smaller.

Technology

Cave sites where P. robustus lived often had tools made from bone, while tools made from stone were linked to early Homo. The first possible bone tool was found in 1959. Later digs found more bone tools, usually next to stone tools made by early Homo. These bone tools might have been used to cut plants, process fruits, strip bark from trees, or dig up tubers and insects. The teeth of P. robustus suggest they didn’t need to bite large pieces of food often, maybe because they used tools to prepare food.

In the late 1980s and early 1990s, studies of bones from Swartkrans Cave found that some had been burned, suggesting fire may have been used regularly. However, it’s also possible these bones were burned naturally in wildfires and later washed into the cave. The earliest clear evidence of fire use dates to 1.7 million years ago at Wonderwerk Cave, linked to _H. ergaster/H. erectus.

Development

Australopithecines, including P. robustus, are thought to have grown faster than modern humans, based on how their teeth developed. The first permanent molar in P. robustus may have appeared between 2.9 to 3.2 years old. While growing, the front of the jaw in P. robustus grew wider, unlike in modern humans where the jaw stays flatter. By the time the first molar came in, the jaw and teeth had grown a lot, making it very strong. Females may have reached full size when their third molar came in, but males kept growing bigger afterward.

Life history

Studies of teeth suggest that P. robustus lived about 17 to 35 years on average. Some scientists think they reached maturity around 11 years old, similar to a middle point between chimpanzees and humans. Estimates based on body size suggest they were born after about 7.6 months of pregnancy, weaned around 2.5 years, reached maturity at 9.7 years, started breeding at 11.4 years, had babies every 45 months, and lived up to about 43 years. These estimates are similar to other australopithecines and chimpanzees, but differ from modern humans.

Pathology

Studies of teeth from P. robustus show they had fewer tooth repairs from wear or cavities compared to modern chimpanzees and gorillas, but more than other australopithecines. Many had spots where enamel didn’t form properly, which might have been a genetic condition. Some had cavities, which is surprising because their diet likely included gritty foods that usually prevent cavities. They also showed signs of gum disease and bone loss around the teeth, possibly due to different diets or chewing habits compared to other australopithecines.

Palaeoecology

The Cradle of Humankind during the time of Paranthropus robustus was home to many animals, including springbok, giraffes, and elephants. There were also big cats like sabertoothed cats such as Dinofelis and Megantereon, as well as hyenas.

Paranthropus robustus lived alongside other early humans like Homo ergaster/Homo erectus and Australopithecus sediba. These species shared the area with each other, though scientists debate how they interacted. The landscape was a mix of open fields and wooded areas, with cooler temperatures than later humans.

Fossil-bearing deposits

Location of the Cradle of Humankind

Fossil-bearing caves (P. robustus is known from 1, 7, and 11–14)

Swartkrans

At Swartkrans, P. robustus fossils were found in Members 1–3. Over 300 P. robustus specimens, mostly teeth, have been found here. Homo species were also present, but their exact identification is debated.

Sterkfontein

At Sterkfontein, only two specimens are firmly identified as P. robustus. These come from a layer dated to between 2 and 1.7 million years ago. Earlier layers had Australopithecus africanus. Some scientists think an earlier specimen might be an ancestor to P. robustus.

Kromdraai

At Kromdraai, P. robustus fossils were found mainly in Member 3. About 31 specimens from at least 17 individuals have been recovered. The fossils here might be older than those from other sites.

Gondolin Cave

Gondolin Cave has yielded three hominin specimens. One is from early Homo, another is a gracile australopithecine, and the third is a robust australopithecine tooth. This tooth is very large, almost as big as those from another robust species.

Cooper's Cave

Cooper's Cave was first reported to have P. robustus remains in 2000. The fossils include a crushed face, teeth, and skull pieces. The animal remains found here are similar to those at Swartkrans and Kromdraai.

Drimolen Cave

Drimolen Cave was first found to have hominin remains in 1992. It has produced 79 P. robustus specimens, including some of the most complete skulls. The site is thought to be between 2 and 1.5 million years old.

Predation

Australopithecine bones in caves might have been brought there by big carnivores. A juvenile P. robustus skull shows marks that look like they were made by a leopard's teeth. Scientists think leopards, sabertoothed cats, and hyenas might have hunted these early humans. Baboons today stay near larger animals for safety, and early humans might have done the same.

Extinction

P. robustus preferred wooded areas and lived during a wet period in South Africa. As the climate changed and dry periods became longer, these wooded areas shrank. This may have led to the extinction of P. robustus, which disappeared around 1 to 0.6 million years ago. Early Homo species may have survived because they lived over a larger area and could find better refuge during tough times.