How did humans get to occupy Middle Earth and where did they come from? No clear answer has emerged so far to the second question. Tropical East Africa has traditionally been cited as the place of origin and the subsequent geographical expansion of humans has been labelled the ‘Out-of-Africa 1’ dispersal. My own hunch, given that the human adaptation was a response to a drying world, is that the area of origin was the core of Middle Earth and not the wet tropics of south-east Asia. That would place the area of origin of erectus somewhere in present-day north-east Africa, the Arabian Peninsula, the Middle East, or the low-lying coastal areas of the Persian Gulf.
As to the first question—how humans occupied Middle Earth—the first thing that we have to have very clear is that it had nothing to do with having elongated hind limbs, being highly mobile, or having large home ranges. I want to clear this matter up first because it has often been cited as the reason for the expansion of the first humans out of Africa. The anatomical and behavioural changes that came with the first humans, and which I described in Chapter 4, permitted their survival and reproduction. These were not features that directly influenced large-scale geographical expansion. They did so indirectly by facilitating the growth of the human population which in turn forced a demographic expansion as home territories were saturated and surplus individuals had to move elsewhere. The first geographical expansion of humans, like that of all other hominids and indeed all other animals, happened over a series of generations and did not involve the migration of long-legged individuals. Long legs were a factor promoting population growth, just as a powerful set of teeth might have been for a hyena.
The Dmanisi hominids were supposedly short and have been compared to habilis, apparently lacking the traits that characterized long hind limbs of humans.8 Given what I have already said, my view is that if so, this is an observation that does not really have a bearing on the geographical expansion of humans or any other hominid, Dmanisi ones included, across Middle Earth. In fact, it now seems that the Dmanisi hominids were not short at all and a comparison with recent hunter-gatherers shows that they were very similar.9 The real difference in stature only lies with some of the largest female early human specimens from East Africa. Those aside, there is a significant overlap in stature among the hominids of 1.7 million years ago, making the separation of those at the lower end of the stature range a difficult task. This problem has a bearing on the study of the brain of the early hominids.
The first humans had large brains but they showed considerable variation. The specimens that have been studied reveal brain volumes that range between 691 and 1,030 cubic centimetres.10 This means that at the lower end the brains of the first humans overlapped with the larger habilis brains and at the upper end they overlapped with the smaller brains of modern people today.11 Factoring body size into the comparisons, Kappelman came to the conclusion that relative brain size increased sharply in the first humans and then remained fairly constant. What did change after our appearance was body size, and brain size increased correspondingly. The only significant change to this trend seems to have occurred much later, around 100 thousand years ago, when there was a decrease in body mass with a correspondingly sharp increase in relative brain size. I will discuss these later developments further on in this book. What is important here is that habilis seems to have started a process of brain enlargement which continued in the first humans simply because they became larger. This means that humans had larger brains than habilis when looked at in absolute terms, though there was some overlap between sizes of the two.
Large brains require a lot of feeding and the enlarged brain of the first humans took a great deal more food than the brains of its predecessors, which it did at the expense of other organs of the body, particularly the gut which was reduced in size.12 But the brain is also a sensitive organ that not only needs physical protection inside a hard skull but which must also be kept within a narrow working temperature range, constant and warm to maintain optimal activity of the neurons, if it is to function properly.13 A core body temperature of 37 oC is optimal for all mammals and birds so it has deep evolutionary importance. To keep a lower core temperature would mean constant sweating and a higher temperature would be too close to the point at which proteins are denatured, risking vital life processes: so 37 oC seems to have been set as the ideal core body temperature.
The early hominids that lived in the hot conditions of tropical Africa would have faced a battle to keep cool and maintain a stable body temperature. Being larger than their predecessors meant they would have had a lower surface-area to volume ratio, proportionately less area of skin for their body mass, so problems of heat loss would have increased. The human solution would have been sweating, with the result that their activities would have been tied to sources of water. It has been suggested that humans needed large brains in order to be able to retain information on the location of water sources, particularly in sporadically occurring periods of extreme drought.14 Certainly present-day Bushmen are unsurpassed among predators or prey in terms of the amount of information that they store and retrieve about the environment which they use during the course of a year—typically an area of 10 square kilometres. They retain an intimate knowledge of the terrain, especially of the water sources that they have previously used.
In Chapter 4 we saw how the appearance of the Acheulian technology seems to have coincided with the first humans. This new technology did not supplant the earlier way in which stone tools were made15 and both have recently been found in the same sites16 which suggests that they may have even been used by the same people but for different purposes. In Chapter 4 I also linked the Acheulian with mobility; these were tools that could be carried from place to place. What is even more interesting is that Acheulian tools almost always occur in association with fresh water.17 We can now put together the early human package, or better still the package that defined the origins of humanity and the making of the improbable primate.
The package had a legacy that came from a rainforest fruit-eater. That legacy provided a brain capable of getting its bearer around the forest, remembering where and when to find trees with ripe fruit. The legacy also provided a grasping hand and a predisposition to stand on the hind legs when moving on horizontal branches. We find these three features in enhanced, even exaggerated, form in the first humans. The association with water came later and probably became indispensable as foods with high water content, such as fruit, were replaced by others that were drier, such as nuts. The change, which may have been initiated by Sahelanthropus or Ardipithecus or one of their generation, meant that drinking fresh water became part of the daily activities of the human prototype.
What types of places did early human populations inhabit? Let us explore the evidence for the early populations and for the pioneers that arrived in various parts of Middle Earth. In Java in south-east Asia, the human presence is as old, if not older, than in East Africa and the colonization of the island would have taken place via land bridges that are now submerged.18 So what were humans doing on Java?
Humans occupied the site of Trinil in Java around 1.5 million years ago. Over 400 thousand fossils of aquatic and terrestrial vertebrates as well as aquatic invertebrates were excavated from this site during the last decade of the 19th century and the first decade of the 20th century and have been used to reconstruct the environment in which humans lived at the time.19 The impressive list included fish, such as perch, catfish, sharks, giant freshwater stingrays, and sawfish, which indicate the presence of lakes, ponds, rivers, brooks, shallow waters, swamps, estuaries, and coastal waters. This conclusion was backed up by a large diversity of molluscs typical of such aquatic environments. The mammals from Trinil also revealed a close connection with aquatic habitats and the flooded meadows and forests that would have been close by. They included strong swimmers like the tiger, the long-tailed macaque, and an extinct elephant.20 The reptiles were all species associated with water, crocodiles, gavials, turtles, and a monitor lizard. St
orks, a goose, and a shelduck added to the reconstruction of an aquatic environment. Put together, erectus in Trinil lived near coastal rivers, lakes, swamp forests, lagoons, and marshes with minor marine influence. This diverse wetland system merged into grasslands on the drier ground. Trinil must have been an Eden for humans, perhaps more so than the drier environments of Africa where water would have been harder to find. Compared to their cousins in Africa, humans in Trinil were laughing.
The Trinil population seems to have been occupying habitats that had been used by the earliest humans on the island, who had entered 400 thousand years earlier during a period of lowered sea levels and found a low-relief lake margin landscape dominated by moist grasslands with open woodland in the driest areas.21 This would not have been unfamiliar, a landscape dominated by water, trees, and open spaces. The climatic conditions, though, would have been different. In Africa, cooling and drying opened the landscape up to a greater extreme than in south-east Asia. Woodland became open savannah and grasslands became desert; water bodies were scattered. In the wetter world of south-east Asia, cooling and drying broke up the rainforest in favour of open woodland, savannahs, and grasslands but there was water everywhere. On top of all this the lowered sea levels, caused by global cooling and water being trapped as ice in polar regions, opened up land bridges between islands. While dry meant stress in Africa, in south-east Asia it meant paradise.
With subsequent warming and sea-level rise the humans became geographically isolated on these islands, which were sufficiently large to permit populations to persist in isolation for a long time. There were populations of the earliest humans (subspecies erectus) on Java as recently as 100 thousand years ago, and possibly even down to 50 thousand years ago.22 With the resumption of rainfall the rainforest took over once more and the erectus population probably shrank in size and would have been confined to the coastal wetlands. But were they really trapped on these islands? If we follow the argument that I have used so far, it was always the marginal and stressed populations that were the innovators. Now, marginal and stress can mean different things to different populations depending on where they are, and when. For humans on south-east Asian islands warm and wet conditions were stressful because they reduced the area covered by open spaces. They lived on the margins of the great rainforest. So they would have been under pressure to find solutions for survival. Dispersal would have offered a way out on the mainland but here there was the open sea and no primate had ever crossed it.
Humans, it seems, did. The island of Flores, famous for the discovery in 2004 of the small hominid that became popularly known as the Hobbit, never had a land connection with either the mainland or any other island. So it was a real surprise when stone tools from Flores were dated to around 840 thousand years ago.23 So how did humans get to Flores? We do not know. One answer would be that they had craft capable of travelling over water that far back but another would be that they drifted accidentally on logs and other floating debris between islands, particularly after storms. Some were able to establish a population by this means, just like many other animals, including monkeys, have done on countless occasions. What this mysterious finding tells us is that humans, who had lived along deltas and estuaries from around 1.8 million years ago onwards, were regular visitors to the coast;24 so much so that they had been exposed to the possibility of, at the very least, occasional offshore drifting. Humans, from their very origins, were not only hominids of inland freshwater bodies but also of brackish and saltwater coastal ones. Our primate was even more improbable than we might have at first realized.
So it seems that humans in south-east Asia and in East Africa behaved in a similar way, living close to fresh water in areas with trees and open spaces, sometimes with rocky outcrops and caves. But they showed an ability to adapt, within these parameters, to differing circumstances afforded by accidents of geography. For example, they were much more coastal in south-east Asia than in Africa. For a long time we had no knowledge of erectus in the huge subcontinent that lay between south-east Asia and Africa but we now have new information that fills this gap in our knowledge. By at least 1.5 million years ago it seems that India was populated by people who were fully conversant with the Acheulian technology, including the manufacture of hand axes and cleavers.25 In south-eastern India at least, a cluster of sites reveal the occupation of a coastal floodplain. Aquatic environments appear to have also been favourites among the first human populations in India.
From a similar time as the Indian sites, around 1.4 million years ago, humans were living along the shoreline of a wetland in today’s Jordan Valley. The site of Ubeidiya was a mix of deep and shallow lakes, marshes, rivers, and wadis.26 The mammal numbers were dominated by a hippopotamus but there were large numbers of deer, pigs, and a macaque which have been interpreted to represent woodland. My own experience of these species is that they are quite versatile and can also live in more open places, especially close to lakes and marshes. Even so there were also gazelles which indicate open patches of vegetation. There were large numbers of catfish as well as turtles and freshwater molluscs. Ubeidiya is another example of humans living in areas close to fresh water with trees and open spaces nearby at a very early stage. The environment they lived in, not surprisingly given the geographical proximity, resembled East Africa more than southeast Asia.
If we go west from Israel along the Mediterranean coast of North Africa we reach the sites of Ain Hanech and El-Kherba in north-east Algeria. These sites are dated to around 1.8 million years ago and represent an early presence of humans in North Africa.27 At this early site humans seem to have been using only the early Oldowan technology. The site, really a complex of sites spread across an area of 1 square kilometre, has been described as an open and dry landscape which had a permanent body of water.28 The sites produced numerous remains of hippopotamus, a species that would have needed year-round water to a depth of at least 1.5 metres, as well as crocodiles and aquatic turtles. The rest of the mammal fauna was dominated by horses, which indicated open, grassy environments where they could graze. The rich community of mammals included carnivores, elephants, antelopes, gazelles, giraffes, pigs, and rhinos. These animals are not forest dwellers, which is why an open landscape has been inferred. They are species, however, that are not restricted to open, treeless plains so it is very likely that wooded savannahs would also have been present. The North African sector of Middle Earth thus provided good conditions for humans who found the combination of habitat elements that seem to have defined the requirements of its lineage.
The earliest evidence from the northern shore of the Mediterranean, the northern fringes of Middle Earth, comes from the site of Sima de Elephante in Atapuerca, northern Spain,29 and from sites around an ancient lake now disappeared, Lake Baza in southeast Spain, which have been dated to around 1.4-1.2 million years ago.30 Whether humans got here ‘the long way round’, that is up through the Middle East and west across the Mediterranean coast of Europe, or instead managed to cross the Strait of Gibraltar from North Africa, is a matter of debate. In the absence of strong evidence for a sea crossing I prefer to stay with the long land route although we should keep in mind that humans (since I am regarding erectus as a subspecies of Homo sapiens) had somehow got to Flores before 840 thousand years ago. As always, we should keep our minds open to all possibilities. Whichever way they dispersed, they had reached this extreme corner of Europe by 1.4 million years ago.
As in Algeria, these early humans are associated with Oldowan technology only. The Acheulian portable kit did not arrive this early, or so we are told. These differences between sites and technologies have led to a variety of interpretations without satisfactory resolution. The answer may lie in the ecology. In Baza, as in Ain Hanech and south-east Asia, humans were living close to large and permanent wetlands so portable technology, needed when undertaking long journeys between water sources, may not have been important. Instead, they may have been happy with making more expeditious tools that served them we
ll on the lake margins and which they did not have to carry over large distances. In tropical East Africa and India, seasonality in rainfall would have generated a greater patchwork of wetlands. In the permanent environments, the Oldowan would have sufficed but in the seasonal ones on the edge, the Acheulian came in handy. Viewed this way, it is easy to understand the co-occurrence of Oldowan and Acheulian in some places and the presence of one or the other in others. Both, according to this interpretation, were practised by humans depending on where they were and what they were doing.
This leaves one question unanswered, that of the late arrival of the Acheulian in Europe. The first recorded European Acheulian is from south-east Spain, not far from Lake Baza, from around 0.9 million years ago.31 That is half-a-million years after the first humans arrived there. How can we explain the lag? I think that the conditions in western Europe, which have always been comparatively humid because of the proximity of the Atlantic Ocean, would have remained relatively wetter than in other parts of southern Middle Earth that had started to experience aridity. If the Acheulian was a response to dry conditions, providing portability when mobility was needed to find scattered water resources, then we would expect this technology to appear later in the more humid regions.
This is exactly what we find in Lake Baza. At its height it would have approached 60 kilometres in length and would have been another of the kind of wetland paradises in which we have already found the first humans. An impressive and diverse mammal fauna lived on its margins: rhinos, elephants, deer, bison, horses, sabre-toothed cats, large hyenas, and dogs. The lake itself held a population of hippos, and lots of frogs and toads have also been recovered, confirming the presence of fresh water. The list will be familiar to us by now, as I am sure it was for Homo sapiens. These humans probably got by without having to move across huge distances. Like Ain Hanech, this large lake would have held permanent water so the human strategy may have simply involved moving along the shoreline or into the lake shore itself at times when waters receded. That living conditions were favourable this far north is confirmed by a study of fossil species which are good climatic indicators: when humans were at Lake Baza mean annual temperatures were more than 4 oC higher than they are there today and mean annual rainfall was 400 millimetres over that of the present day.32 The occupation of the edges of Middle Earth may have been discontinuous and dictated by climate, a topic that we will visit in Chapter 6.
The Improbable Primate Page 7