A. Genus Homo
A. Genus Homo
As the ice ages began three million years ago, our ancestors were the hominins of eastern and southern Africa. They never saw glaciers or even much snow, but their environment was impacted by the cycle of wet and dry climate. Ice ages led to an arid Africa with increased grasslands and deserts at the expense of woods. It is surely no coincidence that in the early Quaternary hominins evolved a long striding gait and became fully terrestrial animals. Grasslands grew to their maximum range around 2 MYA. 1 At the same time and place, we first find fossils classified in genus Homo, the paleontological definition of “human”.
We must always remember that the line between hominins and humans is fuzzy and arbitrary. That being said, of course we are naturally curious about the first beings that we would recognize as human. The earliest species to have been given this title is Homo habilis, which inhabited eastern Africa around 2 MYA. This is a benchmark transitory species, still with Australopithecine size and proportions, but with a larger braincase, more agile hands, and a command of tools. (Habilis means “handyman”.) A facial feature that made H. habilis look more human was a reduction of the snout into a flatter mouth.
The star of this chapter is Homo erectus, “upright man”. Dated conservatively to the interval of 1.8 – 0.3 MYA, this was the longest-living human species of all time. It was also the farthest-ranging of its day. Homo erectus was the species that took the bold new step where no hominin had gone before: out of Africa. 2 Its range eventually expanded to all of Africa, southern Europe, and southern Asia, all the way to China and Indonesia, where famous early discoveries were known as “Peking Man” and “Java Man”. The strictly African version of H. erectus is called H. ergaster.
Homo erectus looked very similar to us in overall size and shape. It was larger than earlier hominins and had a more modern proportion of shorter arms and longer legs. It had smaller teeth and jaws. It was also the first species to sport the uniquely human nose. The projecting nasal bone was a relatively minor alteration of the skull, but it makes immense psychological difference to us when we look at a face and judge it as “human” or “animal”. The main skeletal difference between H. erectus and H. sapiens was the upper skull. Like all apes before it, the erectus skull had a prominent bony brow ridge and essentially no forehead; the top of the head appeared “squashed flat” compared to ours.
The first humans found in Europe are given the name Homo antecessor. Fossils from Spain date to about 1 MYA. A slightly younger European species is Homo heidelbergensis, dated conservatively to 600 – 400 KYA and also found in Africa and southwest Asia. Humans of this period could probably walk among us without a whole lot of notice. It takes expertise to point out the subtle anatomical differences between these early European species and modern humans (they had a flatter, longer cranium and generally more robust build). Heidelbergs were also beginning to exhibit some relatively sophisticated behavior such as building shelters and using spears.
Europe’s famous Neanderthal man (officially Homo neanderthalensis) is known almost entirely from much younger fossils dating to the Chapter 5 time scale. However, DNA analysis recently identified some 430,000-year-old human remains in Spain as early Neanderthals. 3 The same study concluded that Neanderthals and modern humans diverged from a common ancestor around 600 – 800 KYA. 4 This leads to H. antecessor or heidelbergensis as good candidates for the Neanderthal / modern human common ancestor. Neanderthals eventually spread eastward to central Asia. They stuck to high latitudes, apparently thriving near the glacial line as they fed on cold-climate animals.
B. The Multi-Regional Hypothesis
One of the most fundamental debates about early human origins concerns long-term ancestry and migration. Fossil humans – humans that were not quite anatomically identical to us – roamed Africa, Asia, and Europe for two million years. By criteria of skeletal appearance, some scientists have categorized the fossil record into more than ten different human species. Now they are all gone but us Homo sapiens. Why is that?
Here we have to pick at the competing definitions of the word “species”. We can define fossil species by physical features or by phylogenic relationships. Appearances are much easier to measure, but they don’t tell the full story of ancestry. The real question is: how many fossil human “species” were evolutionary dead-ends, and how many belonged to the lineage that still lives on with us?
The two classic answers to this question are hypotheses called Multi-Regional Evolution (MRE) and Recent African Evolution (RAE). Both were refined in the 1980s. In light of evidence since then, they seem to be converging toward a middle ground. Still, contrasting their differences is a good way to frame the issue.
In its extreme form, the MRE hypothesis contends that Homo has always been one species, a widespread but continuous gene pool. 5 Early human species such as ergaster, antecessor, and heidelbergensis were just local variations of Homo erectus. The Old World supercontinent is vast, but it does not pose any distinct boundaries to human mating. Genes continued to flow from one end to the other, and Homo sapiens is the long-term result of this interbreeding across multiple regions. Local idiosyncrasies eventually got ironed out in the universal human. Under this hypothesis, human beings ourselves were the world’s first product of globalization!
The RAE hypothesis (currently the majority opinion) 6 views the Homo genus as several species that did not mate with each other. Homo sapiens was just one of them. After evolving about 100,000 years ago in Africa, according to RAE, our species then rapidly populated the rest of the world and drove the rest of the human genus to extinction. Evidence in support of RAE will be taken up in Chapter 5.
The earliest known Asian humans, dated to 1.8 MYA, were discovered in Georgia in the 1990s – 2000s. 7 The few skeletons found there exhibited so much diversity that they might have been regarded as different species if they had been found in different locations. 8 This realization has prompted scientists to be more conservative about lumping physical variations together into single species. In other words, the number of early human species might be overestimated based on skeletal features alone.
MRE advocates believe in a “regional continuity” of fossil and cultural remains. In each particular region, human fossils bear a certain combination of traits that has never changed. For example, northern Chinese fossil skulls have particular “Chinese” features 1 that appeared the same 500,000 years ago as today. 9 These features were not wiped out by an African invasion. The same is true of tool use; Chinese early humans never adopted the African hand axe. 10 Furthermore, fossils in multiple regions exhibit a gradual long-term transition from erectus or Neanderthal features to sapiens features. 11
The strongest evidence in favor of MRE is nuclear DNA analysis. One study showed an ongoing pattern of gene flow across the Old World for most of the last two million years. 12 Gene flow can occur when one group migrates and mates as it goes, or when communities mate in one gradual continuum across the continent. There was a mass migration several hundred thousand years ago, which included the travels of Homo antecessor / heidelbergensis from Africa into Europe and Asia. 13 This mass migration was characterized by assimilation of new genes into old populations. 14 It is now well known that Neanderthal DNA is not completely extinct, but comprises a small percentage of today’s human genome. 15 Some African / European mating even seems to date back at least 200,000 years, which would be impossible if modern humans only left Africa 100 KYA. 16 These conclusions would seem to close the case on RAE, but MRE is imperfect 17 and of course we have only heard one side of the story. To be continued in Chapter 5 … .
C. The Big Brain Bang
One of the most striking features of the human fossil record is the ballooning brain. Brain size is limited by cranial capacity, the volume of the skull’s hollow interior. Australopithecus species had an average cranial capacity of 450 cc, 2 slightly larger than a modern chimpanzee’s, and their capacity held steady for two million years. In the two million years since, the Homo genus has tripled that volume!
Sheer brain size is not a completely fair measurement, because humans have larger bodies than Australopithecines. Yet even when brain size is measured as a percentage of body mass, this ratio has roughly doubled in the same time frame. 18 The trend is undeniably cast in stone.
The big brain bang raises two obvious questions with not-so-obvious answers: the cause and the effect. The field abounds with hypotheses. Several factors that are commonly cited as causes are also described as effects. This seems sensible; a combination of positive feedback loops could have dramatic consequences. For example, the use of tools can facilitate butchering animals, which in turn can feed a larger brain. If a larger brain is a smarter brain, then it can invent better tools for hunting and butchering … ad infinitum. 19 Likewise, if smarter early humans could outlive and outmate their dimwitted neighbors, they would have more egg headed children, initiating a cerebral arms race. 20
The underlying assumption here, though, is that larger brains are smarter. This is true in a very gross sense (vertebrates are certainly more intelligent than insects) but, on closer comparison, the brain size / intelligence correlation is very weak, especially among individuals within a species. 21 Compounding this, the remarkable swelling of Homo erectus’ brain did not display many immediate signs of intellectual progress. Aside from tools and fire, all the signs of humanity’s remarkable intelligence occurred only within the Chapter 5 time frame. It seems that the brain probably enlarged for different reasons, secondarily acquiring the exceptional potential that was later exploited.
Could it have had something to do with climate? The synchronization of the big brain bang with the ice ages, and with humanity’s expansions out of Africa, is too compelling to ignore. It was around two million years ago that H. erectus first encountered winter weather, during which plants were dormant and humans had to learn how to hunt to survive. Compared to the other apes, humans had by far the broadest range. Perhaps braininess was man’s adaptation to becoming a generalist, able to conquer a variety of niches. Then there were the longer-term cycles of climate change. A species acclimated to harsh ice age weather could flourish explosively in a bountiful interglacial (such as the present one; see Chapter 4). These “boom times” could lead to faster growth and sexual maturity. Some scientists attribute our large head-to-body ratio as a consequence of juvenilization – the carry-over of childlike proportions into adulthood – especially during such times of rapid growth and reproduction. 22
Now that we are endowed with our top-heavy anatomy, we think of it as an obvious blessing. We must remember that every evolutionary gain comes at a cost. Human brains are ridiculously expensive. We spend 20% of our energy on this 2% of our mass. 23 Worse yet, large brains and skulls make childbirth difficult, not an uncommon cause of death for mothers and infants. The solution has been a slowdown of body growth, yet this has resulted in human infants’ being abnormally underdeveloped and helpless. The fact that humans became so brainy despite these serious challenges suggests that there must have been a persistent evolutionary pressure behind the trend.
The modern human brain is not anatomically outstanding by any single metric. 24 It is not the largest brain by absolute or relative size. It does not have the most neurons or the largest cortex. We seem to have gotten lucky with a double whammy: primate brain structure augmented by human brain size. Elephants and whales have larger brains, but primates evolved denser and faster neurons. 25 Then the big brain bang made human brains exceptionally large and complex 26 even for primates.
As discussed in the introduction, the field of archaeology – the study of human artifacts – dates back about three million years. For most of this time, the only artifacts to be found are those made of stone. Early humans may have made tools out of wood or bones, but most of them are long gone. 27 Metal working came much more recently, so the Stone Age is aptly named as the time when stone tools were at the “cutting edge” of technology. In fact, though today we use the phrase “Stone Age technology” as an insult, in its time it was revolutionary.
Chapter 7 introduced tool use as an outgrowth of apes’ special mechanical insight. Chimpanzees commonly crack nuts open with hammer stones and use sticks to catch insects. Hominins probably had a similar toolkit, but they took it to another level when they began making their own stone tools. Chimpanzees (and presumably early hominins) use stones as they find them. Only humans 3 can modify stones to create new special-purpose tools. Functionally, human tools go beyond mere hammering and usually have sharp edges; we are the only animal to use tools for cutting and scraping.
There are multiple reasons that such tools came from humans alone, even aside from advanced intellect. The earliest tools would be especially valuable for butchery, a more urgent need for increasingly carnivorous humans than for their jungle cousins. The earliest evidence of tool usage takes the form of notches cut in animal bone, which bear the distinctive pattern of butchering blades. 28 We can imagine a group of men carving up an antelope carcass quickly before the lions and hyenas arrive, and proudly carrying the meat and leather back home for their family. Furthermore, human hands, with their long opposable thumbs and fine musculature, have a much better precision grip than other apes. Domesticated apes are not very good at making humanesque tools even when they are taught.
It is not clear who the first tool-crafting hominins were. The record was once given to Homo habilis, which was actually defined as human on the basis of tools. However, the archaeological record has now been pushed back past three million years, well beyond H. habilis. Species such as A. garhi and Kenyanthropus platyops are now considered likely candidates for the earliest tool makers, making them “human” by at least one definition!
The basic technique of making stone tools is called knapping. A knapper strikes one stone, the core, with another stone, the hammer. When the core is properly selected and properly struck, the hammer will knock sharp flakes off of it. The best flakes are functional as scrapers and blades, and the core itself can serve as a larger tool. Knapping is an art much more sophisticated than just bashing rocks together. Selecting the wrong stones or striking them together improperly will result in useless shards. A modern hobbyist must spend years mastering the craft. 29 Clearly, our ancestors found tool-making important enough to devote arduous practice to it.
The Lomekwian tools from 3.3 MYA were large and crude by later standards, and have only been found in one location. By 2.5 MYA, tool use was widespread and moderately standardized. Oldowan or “pebble tool” technology used round pebble stones. Tool makers recognized that certain minerals worked better than others, and they learned how to find smooth stones in riverbeds. The Oldowan technique is thus the first evidence of human culture; it seemed to involve a diffusion of knowledge. Homo erectus adopted Oldowan technology and took it out of Africa. Oldowan sites are found from Spain to Korea.It was also H. erectus who advanced tools to the Acheulean stage, which eventually spread almost everywhere except China. The icon of Acheulean technology is the bifacial hand axe, a core stone carefully carved into a hand-sized teardrop shape. Hand axes are very symmetric and deliberately shaped. It is difficult for a layman to differentiate early Oldowan tools from naturally occurring rocks. Acheulean hand axes are very obviously man-made. Looking at one gives a glimpse into a mind with an undeniably human form of conscious agency, stunning for an artifact millions of years old. Tools confer enormous benefits for feeding and defense, so they had quite an impact on the evolution of their makers. As tools took over some functions of muscles and teeth, the entire body became more gracile. The unique attributes of the human hand show strong evidence of selection for grasping objects, throwing or hurling them together, and making precise manipulations – and this evolution happened quickly within the last two million years. 30 It seems that tools made man just as man made tools.
- Thure E. Cerling et al., “Woody cover and hominin environments in the past 6 million years”, Nature vol. 476 (8/4/2011) 51-56 at 55, http://www.nature.com/articles/nature10306 (accessed and saved 2/11/2018). ↩
- Reid Ferring et al., “Earliest human occupations at Dmanisi (Georgian Caucasus) dated to 1.85-1.78 Ma”, PNAS vol. 108 no. 26 (6/28/2011) 10432-10436, http://www.pnas.org/content/108/26/10432 (accessed and saved 2/18/2018). ↩
- Matthias Meyer et al., “Nuclear DNA sequences from the Middle Pleistocene Sima de los Huesos hominins”, Nature 531, 504-507 (3/24/2016), www.nature.com/articles/nature17405 (pay site; cover page accessed and saved 2/18/2018). ↩
- Meyer 2016, as summarized by Ewen Callaway, “Oldest ancient-human DNA details dawn of Neanderthals”, Nature 531, 286 (3/17/2016), https://www.nature.com/news/oldest-ancient-human-dna-details-dawn-of-neanderthals-1.19557 (accessed and saved 2/18/2018). ↩
- Milford Wolpoff, Wu Zhi, and Alan Thorne, “Modern Homo sapiens Origins: A General Theory of Hominid Evolution Involving the Fossil Evidence from East Asia”, in Fred Smith and Frank Spencer, editors, The Origins of Modern Humans: A World Survey of the Fossil Evidence, Liss publishers (New York, 1984) pp. 411 – 483. https://www.scribd.com/doc/56574117 (accessed and saved 2/25/2018; sign-up required). ↩
- David Begun, “The Past, Present and Future of Palaeoanthropology”. In: David Begun, editor, A Companion to Paleoanthropology. Wiley-Blackwell. (2013) p. 8. https://books.google.com/books?id=oIoT1RcFeCwC (accessed and relevant quote saved 2/25/2018). ↩
- Abesalom Vekua, David Lordkipanidze, et al., “A new skull of early Homo from Dmanisi, Georgia”, Science 297(5578) (7/05/2002), 85-9. https://www.ncbi.nlm.nih.gov/pubmed/12098694 (accessed and saved 2/25/2018). ↩
- David Lordkipanidze (discoverer) quoted by Ian Sample in “Skull of Homo erectus throws story of human evolution into disarray”, The Guardian (10/17/2013), https://www.theguardian.com/science/2013/oct/17/skull-homo-erectus-human-evolution (accessed and saved 2/25/2018). ↩
- Milford Wolpoff, Wu Zhi, and Alan Thorne, “Modern Homo sapiens Origins: A General Theory of Hominid Evolution Involving the Fossil Evidence from East Asia”, in Fred Smith and Frank Spencer, editors, The Origins of Modern Humans: A World Survey of the Fossil Evidence, Liss publishers (New York, 1984) pp. 411 – 483. https://www.scribd.com/doc/56574117 (accessed and saved 2/25/2018; sign-up required ↩
- Milford Wolpoff and Alan Thorne, “The multiregional evolution of humans”, Scientific American vol. 13 no. 2s pp. 46 – 53 (May, 2003), http://www-personal.umich.edu/~wolpoff/Papers/Sci%20Am%202003.pdf (accessed and saved 3/03/2018). ↩
- Berhane Asfaw et al., “Remains of Homo erectus from Bouri, Middle Awash, Ethiopia”, Nature 416, 317-320 (3/21/2002), http://www.nature.com/articles/416317a (accessed and saved 3/03/2018). ↩
- Alan Templeton, “Haplotype Trees and Modern Human Origins”, Yearbook of Physical Anthropology 48:33-59 (2005). Stable URL https://www.ncbi.nlm.nih.gov/pubmed/16369961, free copy at http://esa.ipb.pt/pdf/28.pdf (accessed and saved 2/25/2018). ↩
- Colin Groves, “Hominin migrations before Homo sapiens: Out of Africa – how many times?” in Peter Bellwood, editor, The Global Prehistory of Human Migration, Wiley (2015), pp. 21 – 23, https://books.google.com/books?id=TyJlBAAAQBAJ (accessed 2/25/2018). ↩
- Alan R. Templeton, “Out of Africa again and again’, Nature 416, 45-51 (3/07/2002), www.nature.com/articles/416045a (accessed and saved 2/24/2018). ↩
- Richard E. Green et al., “A Draft Sequence of the Neandertal Genome”, Science 328(5979): 710-722 (5/07/2010), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5100745/ (accessed and saved 3/10/2018). ↩
- Cosimo Posth et al., “Deeply divergent archaic mitochondrial genome provides lower time boundary for African gene flow into Neanderthals”, Nature Communications 8:16046 (7/04/2017), http://www.nature.com/articles/ncomms16046 (accessed and saved 3/25/2018). Plain English summary by Ann Gibbons, “Neandertals and modern humans started mating early”, Science (7/04/2017), http://www.sciencemag.org/news/2017/07/neandertals-and-modern-humans-started-mating-early (accessed and saved 2/25/2018). ↩
- Fred H. Smith, Anthony B. Falsetti, and Steven M. Donnelly, “Modern Human Origins”, Yearbook of Physical Anthropology 32:35-68 (1989), http://onlinelibrary.wiley.com/doi/10.1002/ajpa.1330320504/epdf (accessed and saved 3/03/2018). ↩
- I based this calculation on a CC of 450 cc for A. afarensis and 1,350 cc for H. sapiens, and a male-female average mass of 36 kg for A. afarensis and 63 kg for properweight H. sapiens. ↩
- Kwang Hyun Ko, “Origins of human intelligence: The chain of tool-making and brain evolution”, Anthropological Notebooks 22 (1):5-22 (2016), http://www.drustvo-antropologov.si/AN/PDF/2016_1/Anthropological_Notebooks_XXII_1_Ko.pdf (accessed and saved 3/18/2018). ↩
- The “social intelligence hypothesis” grew out of Alison Jolly’s research on primates in general. “Lemur Social Behavior and Primate Intelligence”, Science vol. 153, Issue 3735, pp. 501-506 (July, 1966), http://science.sciencemag.org/content/153/3735/501 (accessed and saved 3/18/2018). ↩
- Javier DeFelipe, “The evolution of the brain, the human nature of cortical circuits, and intellectual creativity”, Frontiers in Neuroanatomy, vol. 5 Article 29 (May, 2011), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3098448/ (accessed and saved 3/18/2018). ↩
- William H. Calvin, The Ascent of Mind, iUniverse.com publishers (Lincoln, NE, 2000), especially Chapter 3. ↩
- Donald Clarke and Louis Sokoloff, “Circulation and energy metabolism in the brain”, Basic Neurochemistry: Molecular, Cellular and Medical Aspects, 6th ed., G.J. Siegel editor, Lippincott-Raven Publishers (Philadelphia, 1999) 637 at 650-651, https://fordham.bepress.com/cgi/viewcontent.cgi?article=1082&context=chem_facultypubs (accessed and saved 3/18/2018). ↩
- Suzana Herculano-Houzel, “The Remarkable, Yet Not Extraordinary, Human Brain as a Scaled-Up Primate Brain and Its Associated Cost”, In the Light of Evolution Volume VI: Brain and Behavior, National Academies Press (Washington, DC, 1/25/2013) Ch. 8, https://www.ncbi.nlm.nih.gov/books/NBK207181/ (accessed and saved 3/11/2018). ↩
- Gerhard Roth and Ursula Dicke, “Evolution of the brain and intelligence”, Trends in Cognitive Sciences vol. 9 no. 5 (May, 2005), https://www.ncbi.nlm.nih.gov/pubmed/15866152 (accessed and saved 3/10/2018). ↩
- Mark V. Flinn, “Evolutionary Anthropology of the Human Family”, Ch. 2 of The Oxford Handbook of Evolutionary Family Psychology, Todd K. Shackelford and Catherine A. Salmon, ed., Oxford University Press (New York, 2011), 12 – 32. On p. 13, Flinn enumerates ways in which the human brain has grown more complex as well as large. ↩
- For example, see J. Lesnik and J.F. Thackeray, “The efficiency of stone and bone tools for opening termite mounds: implications for hominid tool use at Swartrkrans”, South African Journal of Science 103 (Sep – Oct 2007), 354-356, http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-23532007000500002 (accessed and saved 3/25/18). ↩
- Shannon McPherron et al., “Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia”, Nature 466, 857-860 (8/12/2010), http://www.nature.com/articles/nature09248 (accessed and saved 3/25/18). ↩
- Deborah Olausson, “The Use and Abuse of Experimental Flintknapping in Archaeology”, in H. Nami (ed) Experiments and Interpretation of Traditional Technologies: Essays in Honor of Errett Callahan (Lund University, 1/1/2010) pp. 37-56 at 37-38, http://portal.research.lu.se/ws/files/5536991/2425795.pdf (accessed and saved 3/24/18). Corroborated by personal correspondence with expert knapper Thomas Schorr-kon (2018), https://www.youtube.com/watch?v=FA2SNM9ueP4&lc=z22agn2arxfhxphjo04t1aokgrnbbemkvxvutexixv5mrk0h00410.1525729724968289 ↩
- Carol Ward et al., “Early Pleistocene third metacarpal from Kenya and the evolution of modern human-like hand morphology”, PNAS vol 111 no 1 (1/07/2014), 121-124, http://www.pnas.org/content/111/1/121 (accessed and saved 3/25/18). ↩
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