EvC Forum: Great Debate: The argument for placing Homo erectus in the genus Homo

In a discussion of Hominin and Hominid ancestral relationships with the other Great apes, it was argued by Mazzy that Homo erectus did not belong in the genus Homo, while Homo ergaster (specifically ‘Turkana Boy’, did belong in our genus. Rather confusingly, Mazzy later recanted her previous assertion that Homo ergaster belonged in our genus and stated that ‘Turkana Boy’ had, in her words, ..an ape head. While nothing in science prevents someone from changing their mind, any assertion does need to be supported, and Mazzy failed to provide sufficient support for either of her contradictory assertions in that chain of posts. Correspondingly, Mazzy was asked if she would like to enter in a great debate and provide support for her later hypothesis regarding Homo erectus and Homo ergaster. It would not be fair, though, if I didn’t provide Mazzy with a good argument for why anthropologists place both erectus and ergaster in the genus Homo and not with the other great apes.

One of the criterions for being placed in the genus Homo is obligate bipedalism through our specialized post-cranial morphology. This makes sense because we happen to be obligate bipeds, and if evolution is to make sense, then having this derived feature would point to some kind of ancestral relationship. So, what does it mean when we say our genus is obligate bipedal? Basically it means that our skeletal morphology is developed for bipedalism and that we are inefficient and have a hard time using other forms of locomotion, such as knuckle-walking, or arborealism. With that in mind, habitual bipedalism should show itself both in the robustness of the lower limbs but also in the morphology of the species in question. As Homo sapiens is the only surviving species in our genus, we should be able to make relative comparisons between the bone structure of Homo erectus and us versus any of our closest knuckle-walking and arboreal relatives, the panins, the pongoids, and gorilla’s. These comparisons are integral to determining the phylogenic relationships of any fossil and whether a species belongs in our genus. The fact is, Homo erectus’s fossil remains show overwhelming and consistent evidence that they were habitually bipedal.

Considering this, we should look at the osteological argument for the placement of both Homo erectus and Homo ergaster in the genus Homo first, starting from the ‘bottom’ of the skeleton and ending with the cranium. This, to me, seemed appropriate as Hominin evolution has been recognized by most anthropologists, if not all, as not ‘starting from the brain’. Secondly, I will review the very important cultural arguments for placing both Homo erectus and Homo ergaster in the genus Homo. For the purposes of both of these arguments, I will assume that they are actually one species, Homo erectus or Homo erectus sensu lato, as I believe it would have little bearing on the argument last put forth by Mazzy (as I understand it). It should be noted that the scientific consensus is for their placement in our genus (obviously), with a minority placing Homo erectus and Homo ergaster within our own species Homo sapiens (Wolpoff et al, 2000).

So let’s start.

As many are aware, fossilized foot remains for Homo erectus are rare. This shouldn’t be a problem considering that we have some important post-cranial fossil remains from the talus and up, and taking those into consideration, it would make sense that the foot should be adapted for a load-bearing bipedal gait. What we do have are fossilized foot-prints that we can attribute to Homo erectus based on their measurements and dating. It is obvious from these remains that Homo erectus had a modern bipedal gait by roughly 1.5 million years at least.

When we look at the talus and distal end of the tibia of Homo erectus we see that the distal end of the tibia is much more robust than that of the quadrupedal/arboreal great apes, and the perpendicular angle of the tibia in relation to the talus shows that they are definitely structured for load-bearing bipedalism.

Another obvious indication of bipedalism is seen at the angle where the femur and tibia meet. In order to center the load of the body over each foot as we walk and thus provide a stable platform for efficient walking, the angle where the femur and tibia meet creates what we call a ‘valgus’ knee. This knock-knee and corresponding load-bearing walk shows itself in the proximal surface of the tibia. The medial and lateral proximal condyles of the tibia are approximately equal in size, a result of the load bearing, and show a distinct anteroposteriorial elongation as compared to quadrupeds. Measurements of the KNM-WT 15000 tibia length versus the femoral length indicate an adaptation to living in a tropical environment and are comparable to human crural indices (Susan Anton, 2003), while extant tropical apes have much shorter tibia length versus their femur.

Measurements of the tibia and femur also show the robustness, necessary for an obligatory biped. In fact, cross-sectional measurements for the KNM-WT 15000 fossil (page 94) falls well within the human distribution for femoral and humeral strength and very far from that of our closest living relative, the chimpanzee. (Also see Early hominin limb proportions)

We can see the consequences of these shapes in the lower skeleton of Homo erectus when we compare the load-bearing walk of erectus versus earlier hominins such as AL 288-1. Homo erectus load-bearing is very similar to that of and Homo erectus had a consequently, very efficient bipedal gait.

Note the difference between the human and chimpanzee tibia surfaces.

Note the width of the tibia proximal surface and the angle of the femur’s distal condyles.

When we view the knee of Homo erectus we can immediately note the bicondylar angle and the appearance of the ‘valgus’. This is just one of the more strikingly obvious differences that can be seen between those of Homo erectus and Homo sapiens versus the pongoids, panins, and gorilla’s.

Note the lack of valgus in the chimpanzee, versus the human

As we continue on up to the femur of Homo erectus we can see that the femoral head angle and neck shaft length is a result of the need to form a stable inverted pendulum swing for efficient bipedalism and a corresponding shift in the musculature of the hips. The femoral angle from condyle to hip-joint of Homo erectus is about 125 and falls well within the average of Homo sapiens at roughly 115-132 and well outside of that seen in the quadrupedal great ape femurs which have angles of approximately 180. While the femoral neck length is longer in Homo erectus than that seen in later Homo, it is likely due to the smaller brain case of Homo erectus infants during birth which did not require the infant to rotate its head to fit through the birth canal and therefore provided for a more efficient gait. Comparisons of the torsional strength of the femoral subtrochanteric shaft vs. femoral head breadth between Homo erectus and African Apes shows that Homo erectus was well within the range of modern humans and far from the quadrupedal apes. Investigation into the biomechanic implications seem to indicate that Homo erectus had a more efficient bipedal gait than Homo sapiens does and its bipedalism was fully modern.

One of the more distinct differences we can notice between Homo erectus and the quadruped apes is the shape of the pelvis. In fact, the shape of the pelvis of Homo erectus is so obviously designed for bipedal locomotion, that no serious anthropologist has questioned that determination. Why? Compare the pelvis of us, and Homo erectus and then compare them with a quadruped ape below that. (Notice the wide and thick iliums of modern humans and Homo erectus versus the narrow ilium of the chimpanzee).

While the vertebral column of Homo erectus shows some differences in having smaller lumbar bodies (though larger than seen in the quadruped/arboreal apes) and a smaller thoracic canal than seen in Homo sapiens, it shares similarities such as a widening of the spinal canal in the cervical and lumbar bones. A crucial differences between the great apes and Homo erectus (and us) is the s-shaped curve of the backbone, crucial to moderate stresses on the back during walking, and the great apes straight back designed to hold up the rib cage during arboreal suspension.

Next up is the rib-cage. Homo erectus’s thorax is barrel-shaped and is very similar to that of modern Homo sapiens which are both distinct from the funnel shaped thorax of today’s great apes. (Note the shape’s of the rib cages)

First, Modern Human versus Chimp

Next, Homo erectus versus Modern Human

As this first post is getting rather long, I’ll make some short points regarding the upper limbs of Homo erectus and be finished with my descriptions of the post-cranial remains and move on to the cranium. First off, the intermembral and brachial indices most resembles modern humans vice the other great apes, as does the lack of robusticity and lack of curvature of the upper limb bones. The hand phalanges of homo erectus also lack the curvature seen in arboreal and knuckle-walking apes. Conclusively, the morphology of Homo erectus’s upper limbs point to a species that is not arboreal or quadrupedal and is, in fact, habitually bipedal.

Now we can move on to the cranial and a few dental characteristics. The skull is where we can see the biggest differences between modern humans and Homo erectus. That isn’t to say that we don’t share similarities; such as in the relative lack of robusticity of the jaw, as compared to the other great apes and the relative size of our dentition. A change in the diet toward one that relies upon more meat is one of the reasons for this shift to a less robust jaw form. It is also apparent in the dental remains, as Homo erectus molars show wear patterns consistent with a diet largely consisting of meat. While chimpanzees will pursue meat resources on occasion, it is not at the same degree as we see in Homo erectus. Another similarity is the anterior placement of the foramen magnum versus the more dorsal placement seen in the other great apes. This inferiorly placed position of the foramen magnum is an indicator for bipedal locomotion, especially among primates. Homo erectus’s dental arcade is also parabolic, like modern humans, and is less prognathic than previous hominin species. The dental and mandible point to a diet similar to that of modern humans. In the photos below you can see that the other great apes have a U-shaped dental arcade, in contrast to either Homo erectusor ours. (Note also the lack of large canines in both the Homo erectus and modern human dental arcades.)

Homo erectus

Homo sapiens

Pan troglodytes

You should also not the almost complete lack of a diastema in the Homo erectus and Homo sapiens mandibles, as Homo erectus and modern humans have canines that have been reduced to a size just about equal, if not completely equal, to the incisors. Contrast this with the obviously larger canines of the other great apes.

If we compare the brain-size averages of Homo erectus versus modern humans and the other extant great apes, we see that Homo erectus averaged about 970 cc. Homo sapiens average about 1350 cc and gorillas average about 500 cc. Homo erectus is roughly 100% larger than the gorillas, about 50% larger than earlier hominins and about 70% of the modern human average. In fact, brain size increases over time with Homo erectus and we see cranial remains of a younger geological age with brain sizes approaching/exceeding 1100 cc. But when we dig a bit deeper (pardon the pun) we see that brain endocasts of Homo erectus have a strongly developed left-occipital, right-frontal petalia pattern of the brain, indicating possible lateralization, a condition similar to modern humans, which is not seen to the same degree in the other apes.

This next criterion for placement in the genus Homo is the use of technology and cultural adaptation. Homo erectus has been associated with Oldowan technology, or Mode 1 tools, early in its evolution, and shortly after first appearing on scene we start seeing Homo erectus using the more refined Mode 2 tool technology, or Acheulean hand axes. The development of the Acheulean hand axe by Homo erectus was a remarkable achievement and no simple feat, as can be seen in the photos below. While chimpanzees have used basic stick tools to collect termites, leaves to collect water, and stones to crack nuts; I would challenge anyone to find evidence of flint knapping by either chimpanzee, the pongoid, or Gorilla species.

Oldowan technology (Mode 1 tools)

Acheulean technology (Mode 2 tools)

The question of whether Homo erectus belongs in our genus, has hopefully by now, been shown to be an overwhelming Of course.

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