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p. 243. Fossil hominins: their discovery and contextlocked

  • Bernard Wood

Abstract

‘Fossil hominins: their discovery and context’ considers the lines of evidence that can be used to investigate what the hominin clade of the tree of life looks like. The chances of a dead hominin leaving any fossil record are incredibly slim, and even then these fossils are very fragile. Often erosion is required for fossils to be exposed. The chemical profile of the rocks, as well as the various layers of rock, can be used for dating. Absolute dating dates the rocks themselves, with techniques such as radiocarbon dating. Relative dating relies on comparing environments with other fossil sites. Climate change has affected human evolution and can also be reconstructed.

As explained in Chapter 1, a hominin is the label we give to anatomically modern humans and all the extinct species on, or connected to, the modern human twig of the Tree of Life. In this chapter I discuss what the hominin fossil record consists of, how it is discovered and how it and its context are investigated.

The hominin fossil record

A fossil is a relic or trace of a former living organism. Only a tiny fraction of living organisms survive as fossils, and until people were buried deliberately, this also applied to hominins. We are almost certain that the fossils that do survive are a biased sample of the original population, and I discuss the implications of this in more detail in the next chapter. Fossils are usually, but not always, preserved in rocks. Scientists recognize two major categories of fossils. The smaller category, trace fossils, includes footprints, like the 3.6 MY-old footprints from Laetoli in Tanzania that I discuss in Chapter 6, and coprolites (fossilized faeces). The larger category, true fossils, consists of the actual remains of animals or plants. In the hominin fossil record they so outnumber trace fossils that when we use the word fossil it will normally apply to true fossils. Animal fossils usually consist of the hard tissues such as bones and teeth. This is because hard tissues are more resistant to being degraded than are soft tissues such as skin, muscle or the gut. Soft tissues are only preserved in the later stages of the hominin fossil record: for p. 25example, the Bog People found in Denmark and elsewhere in Europe.

Fossilization

The chances that an early hominin’s skeleton would have been preserved in the fossil record are very small. Carnivores, such as the predecessors of modern lions, leopards and cheetahs, would most likely have had the first pick at the carcass of a dead hominin. After them would have come the terrestrial scavengers, led by hyenas, wild dogs and smaller cats, then birds of prey, then insects and finally bacteria. Within two to three years – a surprisingly short time – these organisms are capable of removing most traces of any large mammal.

For its hard tissues to be preserved as fossils, the bones and teeth of a dead hominin would need to have been covered quickly by silt from a stream, by sand on a beach, or by soil washed into a cave. This protects the prospective fossil from further degradation and allows fossilization to take place. Fossilization of a bone begins when chemicals from the surrounding sediments replace the organic material in the hard tissues. Later on, chemicals begin to replace the inorganic material in bones and teeth. These replacement processes proceed for many years, and in this way a bone turns into a fossil. Fossils are essentially bone- or tooth-shaped rocks. In the meantime the sediments that surround the fossil are themselves being converted into rock. Teeth are already hard and durable in life, but chemical replacement also occurs in teeth.

Diagenesis is the word scientists use to describe all the changes that occur to bones and teeth during fossilization. Fossils from different sites, and even fossils from different parts of the same site, show different degrees of fossilization because of small-scale differences in their chemical environment. When fossils are preserved in hard rocks, and when they are freshly exposed, the fossils are very durable. However, if it is exposed to erosion by wind and rain for p. 26any length of time, fossil bone can be as fragile as wet tissue paper. In these cases researchers have to infiltrate the fragile bone with liquid plastic, or its equivalent, in order to stop the fossil from disintegrating. Obviously, deliberate burial greatly increases the chance that skeletons will be preserved in good condition. It is one of the main reasons why the human fossil record gets so much better about 60–70 KYA.

Most hominin fossils are found in rocks formed from sediments laid down by rivers, or on lakeshores, or in the floors of caves. Generally older rocks (and thus the fossils they contain) are in the lower layers and the younger ones are nearer the surface: this principle is called the law of superposition. However, relative movement of rocks brought about by tension and compression, such as the shearing that occurs along faults in the earth’s crust, can confound this general principle. Sedimentary rocks that form in caves are also prone to being jumbled up in even more complex ways. Water that percolates down from the surface can soften and then dissolve old sediments. This produces Swiss-cheese-like cavities, which are then filled by more recent sediments. So within caves new sediments may be below old ones.

Earth scientists use the appearance, texture and distinctive chemistry of rocks to describe and classify them. For example, they might refer to one layer as a ‘pink tuff ’, or another as a ‘silty-sand’. Just as there are rules for naming new species, there are rules and conventions for naming the strata of a newly discovered sedimentary sequence, and there is the equivalent of a Linnaean taxonomy for rocks.

The layer of rock a fossil was buried in is referred to as its ‘parent horizon’. Hominin fossils found within a particular rock layer are, unless there is obvious evidence that they were deliberately buried, considered to be the same age as that layer. A fossil found embedded in a rock is described as being found in situ. Most hominin fossils, however, have been displaced through erosion from p. 27their parent horizon; these are called ‘surface finds’. In order to reliably connect a surface find to its parent horizon, it helps if the fossil still has some of the parent rock, or matrix, attached to, or embedded in, it. This is why careful scientists never completely clean the matrix from a fossil.

Finding fossil hominins

Where do palaeoanthropologists look for early hominin fossils? In the 19th century Charles Darwin argued that, because the closest living relatives of modern humans, the chimpanzee and the gorilla, were both confined to Africa then it was probable that the common ancestor of modern humans was also likely to have lived in Africa. So, for the past 75 years, and especially the last 50 years, Africa has been a focus of human origins field research. But researchers cannot possibly search all of Africa. Are there particular places where hominin fossils are likely to be found?

Palaeoanthropologists look where rocks of the right age (say back to 10 MYA) have been exposed by natural erosion. Erosion occurs in places where the earth’s crust has been buckled and cracked as large landmasses, called tectonic plates, are pushed together. The area between major cracks, or faults, is forced downward, and the earth’s crust on the outside of the major faults is thrust upwards. This is how the floor and walls of rift valleys are formed. The faults that define the sides of rift valleys are sometimes so deep that the liquid core of the earth escapes through them. When it is under very high pressure, the molten core escapes as in a volcanic eruption, otherwise it ‘leaks’ slowly as a flow of molten lava. Usually volcanic eruptions consist of ash (called tephra), which is rich in the chemicals potassium and argon. Rocks formed from these ash layers are called tuffs. Tuffs provide the raw material for the dating of many East African hominin fossil sites. Tuffs also have a distinctive chemical profile, or ‘fingerprint’, and this allows geologists to trace a single tuff not only within a large fossil site, but also across many hundreds of kilometres from one site to another.

p. 28Sometimes hot volcanic ash falls not on the land but on water, and the holes in the lumps of the volcanic pumice people buy for the bathroom are caused by the air bubbles that form when hot ash falls on water.

Fossils are exposed on the sides and floors of the valleys that form as streams and rivers erode their way through the blocks of sediment that are thrown up at faults. Locations like these are called ‘exposures’, and the places on these exposures where fossils have been found are called localities. In East Africa scientists look for hominin fossils in rocks of the right age that have been exposed by the combination of volcanic activity, called tectonism, and erosion in and around the rift valley. Olduvai Gorge, in Tanzania, is probably the best-known example of a rift valley site where both tectonism and erosion have exposed rocks of the right age.

Early hominin fossils are found in a very different geological context in southern Africa. Here, they are found in caves that form when rain runs through cracks in the limestone. Small cracks expand into big cracks, big cracks become cavities, and cavities coalesce to become caves that then fill with soil washed in from the surface. Leopards use the trees that grow in the entrances of the caves as a place to hide carcasses, and hyenas use the entrances of such caves as a den. Scientists think that most of the hominin fossils found in the southern African caves were taken there by leopards or hyenas, or by bone-collecting animals such as porcupines.

Although Africa is the major focus of fieldwork today, it was not that way until well into the 20th century. Before that time the search for human fossils was conducted in Europe and Asia. Europe was where the first prehistorians lived and worked, so it is to be expected that they would have taken advantage of any opportunity that presented itself in their own region before looking for the fossil remains of our ancestors in more exotic places. Just as in 1871 Charles Darwin predicted that Africa would be the birthplace of humankind, Ernst Haeckel, a prominent German naturalist, in p. 29

3. C. K. (Bob) Brain demonstrating the complex stratigraphy at Swartkrans, one of the southern African cave sites where early hominin remains have been found

1874 suggested that the presence of the orangutan, the only non-African great ape, in what was then called the Dutch East Indies (now Borneo and Sumatra in Indonesia) made that region a likely birthplace for humanity. Two years before the publication of Haeckel’s influential book, the naturalist Alfred Russel Wallace (1872) had included detailed information about the morphology and the habits of the orangutan in his book about the natural history of the Malay Archipelago.

Haeckel’s logic and perhaps Wallace’s vivid descriptions of the orangutan evidently appealed to a young trainee surgeon, Eugène Dubois, for in the late 1880s he took a job in the region so he could look for human ancestors. His most famous find, the top of a brain case of a creature that had brow ridges unlike any seen on modern humans, was recovered in 1891 in the bank of the Trinil River in Java. Not all the human ancestors discovered in Asia were found in sediments cut into by rivers. The famous Peking Man fossils came from a cave at a site now called Zhoukoudian, near Beijing in China.

Teamwork

The teams that nowadays look for hominin fossils in Chad, Ethiopia or Eritrea must include a wide range of experts. In addition to palaeoanthropologists, geologists, dating experts, and palaeontologists who can identify and interpret the fossil remains of the animals and plants found with the hominins, a multidisciplinary team should include experts on the factors that bias the fossil record, and may also include earth scientists who can interpret the chemistry of the soils in order to reconstruct ancient habitats. The team’s members have to travel to remote and sometimes dangerous places where they, along with local hired workers who help search for and excavate fossils, need supplies of water, food, and fuel. Leaders of expeditions must have good organizational skills in addition to their scientific qualifications. Big expeditions to inaccessible Central and East African fossil sites are expensive to mount, with the largest ones having annual budgets of tens of thousands of dollars. The southern African cave sites are mostly much more accessible. The majority lie within an hour’s journey time by car from Johannesburg or from Pretoria. This enables scientists to supervise research while working in universities and museums in nearby cities.

Fossils rediscovered

Some dramatic hominin fossil discoveries are made in museums. It is always worthwhile going through the collections of ‘non-human’ fossils recovered from a hominin fossil site. Even the best palaeontologists can miss things as they sort through hundreds of bone fragments. In the past when important hominin discoveries were made they were sometimes sent away to experts for their assessment, and unless great care is taken specimens can be muddled or mislabelled. For example, records show that when a remarkably complete skeleton of a Neanderthal baby was recovered from the site of Le Moustier it was sent to Marcellin Boule for an assessment of its age. However, all trace of the p. 31skeleton seemed to have been lost until a researcher found the bones of a neonate among the stone tools from the site of Les Eyzies! Fortunately, some of the bones were still in their original matrix and this matched rocks in the Vezere River, which runs past Le Moustier.

Dating hominin fossils

Geologists can usually work out the temporal sequence of fossils within a small fossil site. But how do you compare the ages of fossils found at localities hundreds of kilometres apart, and how do you compare the ages of fossils from sites on different continents? To answer these questions you need dating methods. These are divided into two categories, absolute and relative.

Absolute dating methods are mostly applied to the rocks in which the hominin fossil was found, or to non-hominin fossils recovered from the same horizon. Researchers must take great care to preserve the evidence that links a fossil to a particular rock layer. Absolute dating methods rely on knowing the time it takes for natural processes, such as atomic decay, to run their course, or they relate the fossil horizon to precisely calibrated global events such as reversals in the direction of the earth’s magnetic field. This is why absolute dates can be given precisely in calendar years. The best known of these absolute dating methods, radiocarbon dating, is only appropriate for the later stages of human evolution. After 5,730 years (plus or minus 40 years) half of the carbon 14 there was when the organism died has been converted to nitrogen 14 (this is why this length of time is called its ‘half life’). Radiocarbon dating has been used successfully for dating H. sapiens fossils from Australia and Europe, but radiocarbon dates older than 40 KY are unreliable because the amounts of radiocarbon left are too small to be measured precisely.

Most of the hominin fossils from East African sites such as Olduvai Gorge in Tanzania, Koobi Fora in Kenya, and Hadar in Ethiopia, are p. 32from horizons sandwiched between layers of volcanic ash, or tephra, that are rich in isotopes of potassium and argon. Because radioactive potassium and argon convert (or decay) into their daughter products more slowly than carbon 14, potassium/argon and argon/argon dating methods can be used on rocks that contain fossils and stone tools from the early (older than 100 KY) part of the hominin fossil record.

Palaeomagnetic dating uses the complex record of reversals of the direction of the earth’s magnetic field. For long periods in its history the direction of the earth’s magnetic field has been the exact opposite of what it is now. The contemporary direction is called ‘normal’ and the opposite one ‘reversed’. Currents in the liquid core of the earth cause these shifts in the direction of the magnetic field. When the suspended particles settle prior to forming a hard sedimentary rock, minute amounts of magnetic metal in the particles mean that each of them behaves like a magnet. When they settle they line up with the direction of the earth’s magnetic field at the time, and give the rock as a whole a detectable magnetic direction, or polarity. Researchers compare the sequence of changes in magnetic direction preserved in the hominin fossil-bearing sediments with the magnetic record preserved in cores taken from the floor of the deep ocean (called palaeomagnetic columns) and try to find the best match. Some sequences are seen more than once in the reference column, so it helps if another absolute dating method can be used to show researchers which part of the palaeomagnetic record they should focus on. A long period of palaeomagnetic stability is called a ‘chron’, and a relatively short-lived change in magnetic field direction within a chron is called a ‘subchron’. Olduvai Gorge was the first early hominin site to be dated using magnetostratigraphy, and when subchrons were named and not numbered as they are now one of them was called the ‘Olduvai Event’.

Another group of absolute dating methods called amino acid racemization dating uses biochemical reactions as a clock. For p. 33

4. Some of the methods used to date fossil hominins and the time periods they cover

example, eggshell contains an amino acid called leucine. When a shell is formed initially all the leucine is in the L-form. However, over time this L-form of leucine converts, or racemizes, at a more or less steady rate to an alternate version, called the D-form. Thus, the ratio of the two forms, plus the rate of conversion, provides a date for when the shell was formed. Many later African hominin fossil sites contain fragments of ostrich eggshell, and if we make the reasonable assumption that the eggshell in a horizon is the same geological age as any hominins it contains, then ostrich egg shell (OES) dating can provide a potentially useful method. Ostrich egg shell dating is one of several methods (others are electron spin resonance, ESR, and uranium series dating, USD) scientists use to date hominin fossil sites that are between the ranges of radiocarbon and potassium argon dating. These methods are particularly useful for dating sites between 300 and 40 KYA.

p. 34Relative dating methods mostly rely on matching non-hominin fossils found at a site with equivalent evidence from another site that has been reliably dated using absolute methods. If the animal fossils found at Site A are similar to those at Site B, Site A can be assumed to the approximately the same age as Site B. Compared to absolute dating methods, relative dating methods only provide approximate ages for fossils. The use of animal remains for dating, called ‘biochronology’, has been especially important for dating early hominin fossils from the southern African cave sites. Nearly all of these sites contain antelope and monkey fossils. Because the same animals have been absolutely dated at key East African sites, researchers can apply these dates to the layers that contain equivalent fossils in the southern African caves. Biochronology has also been used to date hominin fossil sites in Chad and at Dmanisi, in Georgia.

Dendrochronology, the use of tree rings for relative dating, has been used to improve the precision of carbon dating. Annual tree rings are so reliable that they have been used to correct carbon dates that have been affected by recent human-induced, or anthropogenic, changes in levels of carbon isotopes in the atmosphere.

Reconstructing past environments

Just as the contours of the earth’s surface are different than they were several million years ago, past environments in a region are not necessarily the same as those we see today. Researchers reconstruct past environments using geological and palaeontological evidence. Chemical analysis is used to tell whether a soil was laid down in moist or dry conditions. Palaeontologists can tell a lot about the palaeohabitat from the types of animal fossils that are found along with the fossil hominins. They use both large mammals and small micromammals (such as mice and gerbils) to reconstruct past environments. Small micromammals are especially useful because their geographical ranges are more restricted than p. 35those of larger mammals, so they are likely to provide more precise habitat reconstructions. Fossilized owl pellets are a good source of information about micromammals because owls hunt small mammals within a relatively small range. Researchers who use larger mammals like primates to reconstruct past environments have to be careful not to assume that the habitat preferences of the ancestors were like those of their modern-day representatives. For example, although modern colobus monkeys are mainly leaf eaters who live in dense woodland, their ancestors lived in more open habitats, so the presence of colobus monkeys at a 5 MY-old site does not mean the same as finding contemporary colobus monkeys.

Global climate change

Hominin evolution has taken place at a time when there have been major changes in world climate. Researchers study climate change by looking at deep-sea cores. Microscopic organisms called foraminifera (usually shortened to ‘forams’) are suspended in the water of the world’s oceans. These foraminifera take up two forms of oxygen isotope: one of them, oxygen 16, is lighter, the other, oxygen 18, is heavier. When global temperatures are higher more of the lighter oxygen evaporates, so the ratio of the light to the heavy form reduces: the opposite occurs when global temperatures are cooler. Researchers use the proportions of the two oxygen isotopes to track the temperature of the oceans, and they use ocean water temperature as a proxy for global climate. But the climate in a region is the result of a complex interaction between global climate and local influences such as latitude, altitude, and the presence of mountain ranges.

During the period from 8 to 5 MYA the earth experienced the beginning of a long-term drying and cooling trend. Early hominin evolution took place in Africa at the time of these climatic changes, and the possible influence of climate change on the origin of the hominin lineage will be explored further in Chapter 5.

p. 36

5. Plot of oscillations in oxygen isotope levels during the past six million years, showing that since 3 MYA the global climate has shown a general cooling trend

Later in hominin evolution cyclical changes in global climate, measured using deep sea cores, were superimposed on the long-term cooling trend. Prior to 3 MYA global climate was subject to 23 KY hotter/drier and cooler/wetter cycles. Around 3 MYA the periodicity of these cycles switched to 41 KY and 1 MYA it switched yet again to a 100 KY cycle. These 100 KY cycles are the ones responsible for the periods of intense cold recorded in the northern hemisphere during the past million years. These long cycles had another important impact on human evolution because when so much ice is locked up in the icecaps at the north and south poles, it is inevitable that the sea level will fall. This would have exposed much of what we call the continental shelf. Reductions in sea level of this magnitude allowed modern human ancestors to migrate from the Old World to both Australasia and the New World.