p. 374. Fossil hominins: analysis and interpretation

Classification and taxonomy

Western science classifies all living things according to a scheme devised in 1758 by the Swedish naturalist Carolus Linnaeus. The basic unit of the scheme is the species, a group of morphologically similar animals that consistently breed productively with each other. Individual living animals all belong to a species, similar species are grouped into genera, genera are grouped into tribes, tribes into families, and so on up to categories like kingdoms. Modern humans, Homo sapiens, belong in the species sapiens, the genus Homo, and the tribe Hominini.

A subdiscipline of classification, called ‘nomenclature’, is devoted to prescribing how names should be used in the Linnaean system. There is a formal code for regulating nomenclature, and scientists who think they have discovered a new species must follow this code. Rules in the code govern the types of name that can be given to a new species or genus. For example, the names of commercial p. 38↵products are prohibited: Burgerking ipodensis would not be an acceptable binomial for a new hominin species. It is also important to make sure that the name of an existing taxon is not inadvertently used for a new taxon, otherwise they will be confused.

When researchers decide to introduce a new species, they have to choose one fossil as its ‘type’ specimen. Usually a relatively well-preserved fossil is selected from among those found at the time of the initial discovery: it does not have to be a typical (i.e. an average) member of the species. The significance of the type specimen is that the taxon name is irrevocably attached to it. So, for example, if the type specimen of Homo neanderthalensis was found to be different from all the other fossils included in H. neanderthalensis, then they would have to be assigned to a new species, and it would need to be given a new name. The name H. neanderthalensis cannot be used independently of the type specimen; where it goes, the name goes, too. If researchers eventually decide that a particular specimen should be moved to a new species, then it takes its species name with it. Age counts in nomenclature: if two type specimens end up in the same species, the oldest name is the one that has to be used.

A species is an example of a taxon. All the Linnaean categories are taxa, but when researchers write about ‘a taxon’ they are usually referring to a species. How species are arranged in an increasingly inclusive hierarchy (i.e. larger and larger clusters of species) is called a taxonomy, literally a ‘scheme for taxa’. Taxonomic analysis is the process of determining what taxon hominin fossils should be put in. First, researchers have to decide whether a newly found fossil belongs in an existing hominin taxon. Only if they are convinced that it cannot be assigned to an existing species can they begin to think of making a new species with a new name. The same principles apply all the way up the Linnaean hierarchy, so researchers should only establish a new genus if they are convinced the new species cannot be accommodated in any of the existing hominin genera, and so on up the Linnaean hierarchy.

p. 39Taxonomic analysis and the other methods of analysis described below are based on a detailed assessment of the morphology of a fossil. Its morphology, or phenotype, is what the fossil looks like, both externally and internally. Morphology can be gross morphology, which is what the eye can see unaided, or microscopic morphology, which is what can be seen with a variety of types of microscope. Researchers prepare detailed qualitative descriptions of the size and shape of the fossil, but they also try to capture that information in the form of measurements as a quantitative description. In its simplest form quantitative descriptions consist of distances between defined anatomical landmarks on the fossil: these are called linear measurements. Laser beams and other technologies borrowed from medical imaging now allow researchers to capture details of the external morphology and the internal structure of fossils much more precisely than in the past. For example, Glenn Conroy, a palaeoanthropologist, and Charles Vannier, a medical imaging specialist, both from Washington University in St Louis, pioneered the use of computerized tomography (or CT) imaging to study the internal structure of a fossil hominin cranium from Taung in southern Africa. Subsequently Frans Zonneveld, a medical imaging specialist from Utrecht, and Fred Spoor, a palaeoanthropologist from University College London, further developed these methods so that they can now provide information about the inner ear. Researchers use these data to help sort hominin fossils into species and to reconstruct their posture and hearing.

Researchers must be sure the measurements made on fossils accurately reflect the size and shape of the bone or tooth before it was fossilized. Bones and teeth crack if they are exposed to daily cycles of heating and cooling. Rock matrix gets inside the cracks and artificially enlarges the dimensions of a bone or tooth. Likewise, if a fossil bone is exposed on the surface of the ground in dry and windy conditions both before and after fossilization, sand grains carried by the wind have a ‘sandblasting’ effect and remove part of the outer layer of cortical bone. This erosion artificially reduces the p. 40↵size of the fossil bone. The measurements and the non-metrical morphology of a newly recovered fossil are compared with those of similar specimens in existing fossil taxa. Closely related living animals (in the case of hominins this means modern humans and the African apes) are usually used as models to help decide how much variation should be tolerated within a single species. But Cliff Jolly, a primatologist from New York University who has spent 30 years studying what happens at the boundary between distinctive groups of baboons, suggests that baboons and their close relatives are in some ways a better analogue for hominin evolution. He points out that not only are baboons more widespread than chimpanzees and gorillas, but they are also similar to hominins with respect to the pattern and timing of their recent evolutionary history.

Reconstructing whole fossils from fragments

Hominin fossils several millions of years old are seldom found in good condition. The brain case and the face are particularly fragile and are easily trampled by hoofed animals and crushed by rocks falling from the roof of a cave. Sometimes just one fragment of the brain case is all that is left of a cranium. In a few cases more is preserved, but if the pieces are tiny it is a challenge to reassemble them. It is like a three-dimensional jigsaw puzzle with lots of sky, no clouds and with no picture to help you. One option is to painstakingly reassemble the pieces by hand, but this can take hundreds of hours even by a skilled anatomist who knows every detail of a skull.

Marcia Ponce de León and Christoph Zollikofer from the Anthropological Institute of Zurich are both experts in a new research area called ‘virtual anthropology’. They have used computer power and advances in software design to devise an alternative to reassembling hominin fossils by hand. The fossil is scanned using a laser and a ‘virtual’ version is displayed on the computer screen. Researchers can then move and rotate each piece p. 41↵in any direction to see if any of the pieces fit. The software also enables a missing piece on one side of the cranium to be replaced by mirror imaging the equivalent piece from the other side. Zollikofer and Ponce de León have recently used these methods to make a virtual reconstruction of the cranium of Sahelanthropus tchadensis, a potential early hominin. Similar software in conjunction with CT scans enables structures buried deep in the bone, like the air sinuses, the bony canals of the inner ear, or the roots of the teeth, to be seen clearly.

Determining age and sex

Even if one has a complete or nearly complete skeleton, determining the sex and developmental age of hominin fossil remains can be difficult. These difficulties are compounded when all that remains are small fragments of a cranium. The age at death of a fossil individual that has finished growing is difficult to determine precisely. Dental development can help determine the age of immature individuals, but once all the teeth are erupted and the roots of the teeth are formed dental evidence is less useful.

The size and shape of the bones and teeth, the extent of muscle markings, and the size and shape of the pelvis (although pelvic fragments are rare in the hominin fossil record) are the usual ways the sex of an individual fossil is determined. The underlying assumption is that because in many non-human primates males are larger than females, then early hominin males were also likely to have been larger than early hominin females. This is one aspect of sexual dimorphism, a term that refers to all the differences among individuals that are related to their sex. However, when you are dealing with a sparse fossil record overall size is not always a reliable guide to sex.

There are also complications if one unthinkingly extrapolates modern human sexual dimorphisms to early hominins. For example, in modern humans many pelvic sex dimorphisms occur p. 42↵because of compromises between the requirements of bipedalism and the need in modern human females for space in the pelvis to give birth to a large-brained infant. The same dimorphisms, however, might not apply to small-brained early hominins who are not bipedal in the same way that modern humans are: their pelves may show a unique pattern of sexual dimorphism.

Species and species identification

The most widely used scientific definition of a species is the biological species concept (BSC) that is linked with the late Ernst Mayr, a distinguished Harvard evolutionary biologist. This suggests that a species is a ‘group of interbreeding natural populations, reproductively isolated from other such groups’. This is all well and good when you can observe living animals, and check who is mating with whom, but it is self-evident that this method will not work when we try to recognize species in the fossil record. However, because members of the same species mate with each other and not with members of another species, they resemble each other more closely than they do individuals belonging to any other species. Thus, in the absence of information about its mating habits, we can use the appearance, structure, and (if any DNA is preserved) the genetic make-up of an individual fossil to help allocate it to a species.

But there are problems when researchers try to apply these methods to the fossil record. The first difficulty is that we do not have complete animals in the hominin fossil record. It is customary to divide the components of animals into two categories, soft tissues, such as muscles, nerves, arteries, and hard tissues, such as bones and teeth. The fossil record for human ancestors is restricted to the remains of the hard tissues, and many of these are just fragments of bones and teeth. So the problem for palaeoanthropologists is how to assign a fossil to a species when the only evidence you have is several worn and broken teeth, or a piece of jaw, or part of a thigh bone.

p. 43The second problem is time. Each species has a history, with a beginning (speciation), a middle, and an end. Species either die out without leaving any direct descendants (extinction), or they become the common ancestor of one or more new ‘daughter’ species. The average fossil mammal species lasts for between one and two million years. During such a long history the appearance of that species is unlikely to stay the same. Random variation and morphological responses to climatic variation will cause it to change. But as long as its members only mate with members of the same species then the species should continue to be distinctive. However, even if a scientist spends their whole career observing just one living species they will have studied that species for just an instant during its existence. So the variation you see in museum collections of skeletons belonging to a modern species that have been collected over the course of a hundred years, or so, is not an appropriate model for deciding how much variation one should tolerate in a sample made up of fossils collected at sites that span several hundred thousand years of time.

A good analogy is of a running race. A fossil is like a single still photograph of a long-distance running race. But a long-lived species may well be sampled several times during its history. Palaeoanthropologists need to work out ways of telling whether they are looking at several photographs of the same running race, or single photographs of several different running races. In the case of human evolution this means looking at collections of modern human, and higher primate skeletons, and then using the size and shape variation within those living taxa as a guide to how much variation researchers should tolerate within a collection of fossils assigned to a single species. If the variation is less than that seen in the living taxa then there are good reasons to conclude that only one species is represented in the collection of fossils. Because of the extra time involved with fossil samples palaeoanthropologists try to make an educated guess about the amount of variation they are prepared to tolerate in their fossil sample before they declare that p. 44↵the variation is ‘too great’ to be contained in a single species. But it is only an educated guess.

Deciding how many species are represented in a collection of early hominin fossils is made more difficult because biological variation among hominins, including fossil hominins, is continuous. Therefore where the boundaries between fossil taxa are drawn is a matter of legitimate scientific judgement and debate. The discovery of new fossils or the introduction of new analytical methods often means that boundaries have to change, or palaeoanthropologists have to reconsider the utility of their categories and labels. A new species should be established only if there are really good grounds for believing the new fossil evidence does not belong to an existing species. There needs to be even stronger evidence to establish a new genus.

Speciation

Some researchers think that new species are the result of gradual change involving the whole population. This interpretation of speciation is called ‘phyletic gradualism’, and the form of speciation associated with it is known as ‘anagenesis’. Others see speciation as the result of bursts of rapid evolutionary change concentrated in a geographically restricted subset of the population. This interpretation of speciation is called the ‘punctuated equilibrium’ model. In the latter model in the long interval between the periods of rapid evolutionary change there should be no sustained trends in the direction of morphological evolution, just ‘random walk’ fluctuations in morphology. Species formation in that mode is called ‘cladogenesis’ and the term ‘stasis’ is used to describe the periods of morphological stability between speciation episodes. Almost all researchers now accept that most of the morphological change involved in evolution occurs at the time of speciation.

In some circumstances speciation may be due to quite large-scale changes in the genotype brought about by rearrangements in the p. 45↵

6. The two main hypotheses ‘phyletic gradualism’ and ‘punctuated equilibrium’ about the timing of the morphological change that occurs during evolution

chromosomes. Researchers have suggested that this may have been the mechanism underlying speciation in higher primates.

Particularly intensive periods of species generation and diversification are called ‘adaptive radiations’. They tend to be associated with an opportunity to exploit a new environment, or when extinctions in other groups mean that adaptive opportunities become available in an existing environment. At times like these some lineages tend to generate more species than others, and they are referred to as being ‘speciose’.

All species, including modern humans, will ultimately become extinct. What is at issue is whether extinctions are determined by the intrinsic properties of a species, or by extrinsic factors such as changes in the environment, or by a combination of the two. These competing hypotheses can be tested in the laboratory by varying the conditions under which rapidly evolving organisms such as fruit flies are kept. It can also be investigated by comparing the fossil record with independent evidence about changes in past climates.

Splitters and lumpers

The taxonomy used in this Very Short Introduction recognizes a relatively large number of hominin species, but not all researchers recognize that many species. Researchers who subscribe to taxonomies that recognize many species are called ‘splitters’. Those who recognize fewer species are called ‘lumpers’. Both groups of researchers are looking at the same evidence, they just interpret it differently. Most disagreements among palaeoanthropologists about how many species to recognize in the human fossil record are due to differences in how they interpret variation. Researchers who stress the importance of continuities within the fossil record generally opt for fewer species, whereas those who stress discontinuities within the fossil record will generally recognize more species. However, when all is said and done, all taxonomies are hypotheses. If scientists explain their taxonomy, then other scientists can reinterpret the evidence in any way they choose, as long as everyone makes it clear which fossil specimens they are allocating to the species taxa they choose to recognize.

Cladistic analysis

Once the taxonomy of a new discovery has been worked out, researchers move on to the next stage. This involves using cladistic methods to work out how a fossil hominin taxon is related to modern humans and to other fossil hominin taxa.

The technical term ‘clade’ refers to all (no more and no less) of the organisms descended from a recent common ancestor. The smallest clade consists of just two taxa; the largest includes all living organisms. Cladistic analysis sorts taxa according to the amount of morphology they share, but the morphology has to be of a particular kind. To be helpful for working out relationships between closely related species, the morphology used must be shared by two or more taxa, but it must also vary within the group under investigation, so that it can be used to break that group up into p. 47↵

p. 51subgroups, or clades. For example, the features that make all higher primates mammals, such as the presence of nipples and warm blood, are no use for sorting out detailed relationships among the great apes. But to go to the other extreme, morphology that is found only in one taxon cannot be used to work out the relationships among taxa.

Two taxa that share specialized morphology are referred to as sister taxa. That pair of sister taxa has its own sister taxon (for example Gorilla is the sister taxon of the Pan/Homo clade) and so on. The branching diagram that results is called a cladogram. The same relationships can be represented in writing by using sets of parentheses for sister groups (e.g. ( ( (Homo, Pan) Gorilla) Pongo) ).

Cladistic analysis works on the assumption that if members of two taxa share the same morphology, they must have inherited it from the same recent common ancestor. This assumption is often justified, but not always. We know that primates, including higher primates, have experienced convergent evolution, a process by which different lineages evolve similar morphology independently. The term homoplasy refers to similar morphology seen in two species but which is not inherited from a recent common ancestor. For example, it is likely that thick tooth enamel evolved more than once in human evolution, thus making it a homoplasy within the hominin clade.

Fossil DNA

The newest form of analysis used to work out how hominin taxa are related relies on the extraction and analysis of DNA. In your family, closely related individuals, for example brothers and sisters, share more DNA than do distant cousins. It is the same for taxa. Individuals within a taxon should, on average, share more DNA than two individuals drawn from different taxa. However, despite the importance of DNA in our lives, fossilization quickly causes nucleic acids to degrade. For example, after 50,000 years, only p. 52↵small amounts of DNA survive, and even this is broken into short fragments. A team led by Svante Pääbo, a molecular biologist from the Max Planck Institute for Evolutionary Anthropology at Leipzig, was the first to recover DNA from a fossil hominin, and I will consider fossil DNA evidence further when I discuss Neanderthals in Chapter 7.

Researchers undertaking fossil DNA analysis must take particular care to prevent and detect contamination. When people handle fossils, they inevitably leave hair and skin cells on the fossil and these are a potent source of contamination. Scientists must make sure they are detecting DNA amplified from the fossil hominin and not DNA from other sources. In a recent study of cave bear fossil researchers detected more than twenty different modern human DNA sequences on a single cave bear fossil. Tens, if not hundreds, of people, will have handled most hominin fossils, especially those found many years ago. Working out which of many DNA sequences recovered from a modern human fossil really belongs to that individual will be a challenge.

7. Comparison of the concepts of clades and grades as applied to living higher primates

Grades

Homoplasy complicates our attempts to sort early hominins into clades. An alternative is to sort hominin taxa into grades. A grade is a category based on what an animal does rather than what its phylogenetic relationships are. So for example, Sport Utility Vehicles is the equivalent of a grade, whereas all the cars produced by the Ford Motor Company, including its range of SUVs, is the equivalent of a clade. Grades may also be clades, but they are not necessarily so. For example, leaf-eating, or folivorous, monkeys are a grade and not a clade because folivorous monkeys from the Old and New World are, respectively, just one component of much larger Old and New World monkey clades. A clade must comprise all the descendants of a common ancestor, not just some of them. Palaeoanthropologists are more likely to agree about grades than clades, but determining the branching pattern of the TOL is something that must be pursued, even if the results are controversial. I will refer to some of these controversies in later chapters.

Functional and behavioural morphology

In addition to analysing fossils in order to classify them and arrange them in a cladogram and then a phylogeny, palaeoanthropologists also use the fossil record to work out the adaptations of hominin species. They do this by trying to reconstruct how individuals belonging to the same taxon lived their lives, and then they pool this information with evidence about habitat and generate hypotheses about how that species is adapted to its environment. Researchers try to learn as much about an extinct animal as they would expect to know about a living one. What did it eat? How did it move around? Did it live in social groups, or was it solitary? Palaeoanthropologists attempt to answer these questions by looking at functional or behavioural morphology.

Functional morphology means looking at a bone or tooth and p. 54↵considering what functions it performs best and most frequently. For example, you would only need curved finger bones if you spent a lot of time holding onto branches, so curved finger bones are a sign that climbing was a part of that animal’s locomotion. The shapes of finger joints and the length of the fingers and thumb also provide clues about how well early hominins could have gripped objects. Holding the shaft of a hammer needs a power grip, whereas the ability to hold and use a small, sharp stone tool uses a precision grip and a different combination of arm, forearm, and small hand muscles. Similarly, the thighbones of animals that bear all their weight on their hind limbs are differently shaped from those whose weight is distributed across all four limbs.

Functional morphology can also help to reconstruct the diet of early hominins. The shape of a tooth reflects what was eaten. Teeth with large crowns, with low, rounded, cusps covered by thick enamel are likely to have evolved to cope with a diet that included tough food, or food that was enclosed in some sort of hard outer coating, like the shell of a nut, that needed to be broken before the contents could be eaten. Scientists use microscopes to look at minute scratches not visible to the naked eye that are on all teeth. Foods like tubers that grow in the ground contain a lot of grit, and this leaves tell-tale gouges on the surface of the enamel. Sometimes teeth get scratched when animals trample them, or when hard sand grains are blown against them. But this type of damage should affect the sides and not just the top, or occlusal, surface of a tooth. When they look for clues about the diet of the early hominins by looking for evidence of any microscopic scratches left by food (called microwear), researchers must make sure that they do not confuse these scratches made after death (post mortem) with the scratches made during the life of the individual (ante mortem microwear).

Direct evidence about the kinds of foods hominins ate comes from stable isotope analysis. This form of analysis measures oxygen, nitrogen and carbon isotopes in fossil bones or teeth and then p. 55↵matches the pattern found in the fossil with the patterns seen in living animals whose diets are known. For example, animals that browse on leaves can be distinguished from those that graze on grass and from those that are primarily carnivores. Using such a method, Julia Lee-Thorp, an isotope chemist working at the University of Bradford’s Department of Archaeological Sciences, and her colleagues have shown that 1.5 MY-old Paranthropus hominins from Swartkrans have stable isotope patterns that could only come from eating meat, thus causing researchers to reconsider earlier views that these hominins were primarily, if not exclusively, vegetarians.

Gaps and biases in the hominin fossil record

Over many decades palaeoanthropologists have accumulated hominin fossils from thousands of individuals going back to between 6 and 7 MYA. While this number may sound impressive, the majority are concentrated in the later part of the hominin fossil record. Besides this temporal bias, the hominin fossil record has other biases and weaknesses. The science of working out these biases and trying to correct for them is the topic of taphonomy. Whereas some of the hardest parts of the skeleton such as the teeth and the mandible are well represented in the hominin fossil record, the postcranial skeleton, that is the vertebral column and the limbs, and particularly the vertebral column and the hands and feet are poorly represented. The relative durability of different parts of the skeleton (for example, mandibles are generally heavier and are made of denser bone than vertebrae) is partly responsible for the differential preservation of body parts. Lighter bones like vertebrae are likely to be swept along in the floods that follow torrential rain, and then carried out into a lake, where they will be mixed in with the fossilized bones of fish and crocodiles. In contrast, heavier bones like skulls and jaws will fall to the bottom of the floodwaters, get trapped in the stones on the bed of the stream or river, and are thus preserved in sediments that preserve the heavier bones of other terrestrial animals.

p. 56Another factor influencing differential preservation is which parts of the carcass predators find most tempting. Leopards like to chew the hands and feet of monkeys and, if extinct large carnivores had similar preferences, then these parts of hominins would be in short supply as fossils. Thus, we know more about the evolution of the teeth of fossil hominins than we do about the evolution of their hands and feet. Body size also has a significant influence on whether a taxon is likely to have a fossil record. Large bodied taxa are more likely to be fossilized than ones with small bodies, and larger individuals within a taxon have a greater likelihood of being fossilized than smaller members of the taxon. There is every reason to think that these same biases affect the hominin fossil record.

Some environments are more likely to lead to fossilization and subsequent discovery than others. Thus, we cannot assume that more fossil evidence from a particular period or place means that more individuals were present at that time, or in that place. It may just be that the circumstances at one period of time, or at one location, were more favourable for fossilization than they were at other times, or in other places. Likewise, the absence of hominin fossil evidence at a particular time or place does not have the same implication as its presence. As the saying goes, ‘absence of evidence is not evidence of absence’. Similar logic suggests that taxa are likely to have arisen before they first appear in the fossil record, and they are likely to have survived beyond the time of their most recent appearance in the fossil record. Thus, the first appearance datum (or FAD), and the last appearance datum (or LAD) of taxa in the hominin fossil record are likely to be conservative statements about the times of origin and extinction of a taxon.

The same reservations apply to the geographical distribution of fossil sites. Hominins almost certainly lived in more locations than there are fossil sites. Environments in the past were often different than the ones we see now: parts of the world we now think of as being unattractive habitats were not necessarily that way in the past, and vice versa.

p. 57Lastly, not all environments are conducive to preserving bones and teeth. Some soils are so acidic that bones and teeth rarely survive. For a long time it was assumed that fossils would never be found in forested palaeoenvironments because of the high levels of humic acid. This turned out to be a fallacy, but there are sites where archaeologists would have expected to see stone tools and bones together, and where they only find stone tools: the bones and teeth were dissolved before they could be fossilized.