5.2: Defining the Genus Homo
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- Jennifer Hasty, David G. Lewis, & Marjorie M. Snipes
- OpenStax
Learning Objectives
By the end of this section, you will be able to:
- Describe the time periods and geological context of the genus Homo .
- Identify some key differences between the genus Homo and Australopithecus.
- Define some of the limitations of and challenges in the classification of hominin species in the genus Homo .
- Explain the concept of encephalization as it relates to early hominin evolutionary development and as a tool for hominin classification.
Putting Homo into Context
Before learning about the hominin species that make up the category genus Homo , it will be helpful to become familiar with the key archaeological time periods with which Homo is associated. The species and cultural developments mentioned below will be explored in greater detail in the sections that follow.
- Lower Paleolithic (from roughly 3 million years ago to approximately 300,000 BCE): This period includes H. habilis and the Oldowan tool industry, followed by H. ergaster and the Acheulean tool industry.
- Middle Paleolithic (approximately 300,000–40,000 BCE): This period includes continued use of Acheulean tools by H. heidelbergensis, followed by H. neanderthalensis and the Mousterian tool industry.
- Upper Paleolithic (approximately 43,000–26,000 BCE): The Upper Paleolithic saw the emergence of cave art like that found in the famous Chauvet Cave in France (Figure 5.29), Venus figurines (Figure 5.28), and an increased use of bone and antler in tools and jewelry. The most recent ice age occurred during this time, with glaciers covering huge parts of the planet. The emergence of Paleoindians and the use of Clovis points, which were used to kill large game such as mastodons and mammoths, occurred near the end of this time period.
- Neolithic (Agricultural Age) (8,000–3,000 BCE): New innovations appear during the agricultural age, or “Neolithic revolution,” as H. sapiens set up permanent settlements. Humans begin to transition from being hunters and gatherers to growing grow crops, owning land, and domesticating animals.
The Challenge of Defining the Genus Homo
The previous chapter introduced the australopithecines, who were diverse in their physical characteristics (gracile and robust), with large jaws and teeth and small brain size. A key characteristic shared by both the australopithecines and the genus Homo is bipedalism. The transition to bipedalism is linked with various anatomical changes, including longer legs, changes in spinal curvature, and the development of arches in the feet to conserve energy and increase balance when walking.
What criteria other than bipedalism might be used to classify a species under the genus Homo ? Many anthropologists have attempted to establish specific criteria to use in determining a classification of Homo. Paleoanthropologists Mary Leakey, Louis Leakey, and John Napier, as well as primatologist Phillip Tobias, were among the first to extensively study the fossils of Homo habilis , considered to be one of the earliest species in the genus Homo . H. habilis had a brain size of around 661–700 cc, which was larger than the australopithecines’, with hands that were capable of the dexterity needed for making tools, due to bone structure changes and a repositioning of the thumb, which allowed for better grip.
The type specimen OH 7 of H. habilis dated between 2 and 1.7 MYA and was found in 1960 at Olduvai Gorge by Jonathan and Mary Leakey. It was described by Louis Leakey in 1964. Type specimen refers to a specimen that serves as the standard for the taxon or classification group for that species. OH 7 is the identification or accession number of this specific specimen and stands for “Olduvai Hominid #7.” The specimen consisted of a partial juvenile skull, hand, and foot bones. It possessed teeth that were much smaller than those of any australopithecine and was possibly in coexistence with the robust australopithecines ( Paranthropus ). Based on an endocranial cast (an imprint of the interior of the brain case), it was determined that H. habilis may have possessed what is called a Broca’s area in the brain. Broca’s area , which includes two Brodmann areas (referred to as 44 and 45), is located in the middle of the left cerebral cortex of the brain and is especially important for speech development (Figure 5.2). Some scientists have suggested that H. habilis started to develop the neural networks necessary for human speech, while others argue that H. habilis probably already had speech.
The postcranial features (skeletal structures in the body other than the skull) of Homo habilis are not as well established, as is the case for many other early hominin fossils. This can be problematic, as many hominin species coexisted with overlapping traits. Likewise, it can be problematic to have postcranial material and not the cranium or skull. The skull often serves as a diagnostic tool when postcranial materials do not provide enough evidence or provide confusing evidence.
Based on their research on H. habilis , Mary Leakey, Louis Leakey, and John Napier proposed the following criteria for classifying Homo : a brain size over 600 cc; a round, globular skull; tool use; reduced prognathism (protrusion of the jaw) and smaller jaws and mandibles; humanlike postcranial features; and feet that are fully adapted for walking (Leakey, Tobias, and Napier 1964). While this list established specific and fairly comprehensive guidance, the diversity of traits and the ways in which they overlapped didn’t always line up with the criteria.
H. habilis has been at the center of several debates regarding their taxonomic position and relationship with other early archaic Homo species. For example, H. habilis was initially believed to have been a direct human ancestor through the lineage of Homo erectus and then modern humans. This viewpoint is now debated and has resulted in a scientific divide between those supporting H. habilis and those suggesting another Homo species, H. rudolfensis , as being the ancestor of H. erectus. H. rudolfensis is an archaic Homo dated to about 2 MYA, which coexisted with other Homo species during that time period. A cranium was discovered in 1972 along Lake Turkana in Kenya by Bernard Ngeneo, a local Kenyan. The specimen was later described by paleoanthropologist Richard Leakey. There is a lot that is not known about this species; scientists are missing postcranial materials, and as of yet no tools have been found. There are hypotheses that propose that H. rudolfensis might be a H. habilis male, exhibiting a larger cranium than that seen in a female H. habilis . Others suggest it is a completely different species. Another controversy centers on tool use. While Homo habilis was long regarded as the earliest hominin to use stone tools, it has been determined, based on evidence of cutmarks, that at least one australopithecine ( A. garhi ) used stone tools before H. habilis , at around 2.6 MYA (Semaw et al. 1997).
While there are still questions as to the phylogenetic relationship of H. habilis and H. rudolfensis , there is general agreement that Homo did evolve from Australopithecus . The timing and placement of the split between Australopithecus and Homo , however, is still debated. H. habilis was determined to not be an Australopithecus due to its smaller teeth, a humanlike foot, and hand bones that suggested an ability to manipulate objects with precision.
One of the main considerations in classifying H. habilis as a Homo and not an Australopithecus was its cranial capacity , which is a measurement that indicates brain size. With some exceptions, cranial capacity can serve as an indicator of where a hominin fossil might belong in the hominin phylogenetic tree. Encephalization refers to a progressive increase in brain size over time. In human evolution, we can observe encephalization beginning with Homo habilis and progressing more rapidly through H. erectus . Encephalization correlates with an increase in behavioral, cognitive, and cultural complexity. Cognitive developments correspond with our ability to construct and form ideas, including the ability think in and communicate via symbolic and abstract language, such as that used in storytelling, ritual, and art. There are always exceptions, however, such as the island-dwelling, small-brained H. floresiensis , who will be introduced later in this chapter. In spite of having a very small brain, H. floresiensis made and used tools and built fires. This discovery has challenged what we thought we knew about the correlation of brain size and cognitive development in human evolution.
The encephalization quotient (EQ) can serve as a good indicator (with some exceptions) of classification within the genus Homo . The encephalization quotient is a calculation arrived at by comparing the ratio between actual brain size (determined with either a mass or volume calculation) and expected brain size. Body size is a factor in these measurements as expected brain size reflects the relationship between brain and body size for a given taxonomic group (Jerison 1973). The larger the brain weight relative to the overall body weight, the more likely that the brain was used for more complex cognitive tasks. Harry J. Jerison (1973) was the first to develop EQ measurements. The formula he used for calculating EQ in birds and mammals is brain mass/0.12 × (body mass) 0.66 . Other formulas have also been proposed, such as EQ = brain mass (11.22 × body mass 0.76) (Martin 1981). While EQ is a strong tool for studying brain size in early hominins, there are always potential margins of error when dealing with fragmentary fossils, and increasingly alternative forms of measurements are being proposed. One study proposes that EQ should no longer be used as a tool in calculating brain size in primates and other vertebrate species, based on the premise that cognitive performance does not depend on body size and so body size should not be included in the formula (Schaik et al. 2021). Other theories consider the number of cortical neurons and neural connections as most important when considering cognitive ability (Roth and Dicke 2012). According to this approach, the density of the cortex is more associated with intelligence than is brain size. These alternate approaches would perhaps better explain those exceptions in the fossil records, such as H. floresiensis . Other interesting research is looking at potential levels of cognition and memory as it relates to levels of tool complexity (Read and van der Leeuw 2008).
In spite of these criticisms, many see EQ measurements as providing fairly consistent results. Modern humans ( Homo sapiens ) have an EQ of roughly 6.0–7.0 (meaning that their brain mass is six to seven times greater than what one would expect to find in a comparable mammal of the same body size). H. erectus has an EQ of 4.0, while for an australopithecine EQ is around 2.5 to 3.0 (Fuente 2012, 227). Figure 5.4 shows increases in average brain sizes for various species over time.