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 grounded animals. Grasslands grew to their maximum range around 2 MYA. 2 At the same time and place, we first find fossils classified in genus Homo, the paleontological definition of “human”. Not quite yet anatomically modern, these were the “early humans”.
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, though, is Homo erectus. 1 Dated conservatively to the interval of 0.3 – 1.8 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. 3 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. Its teeth and jaws were shrinking but still larger than ours. Erectus was the first species to sport the uniquely human nose. The projecting nasal bone was a relatively unimportant alteration of the skull, 4 but it makes immense psychological difference to us when we look at a face and judge it as “human” or “animal”. 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. Below the neck, it had the same skeleton as us but with more robust bones.
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 400 – 600 TYA and also found in Africa and southwest Asia. Humans of this period looked very similar to one another. It takes expertise to point out the subtle anatomical differences among these early Homo species. Heidelbergs exhibited 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 timescale. However, DNA analysis recently identified some 400,000-year-old human remains in Spain as early Neanderthals. 5 Neanderthals and modern humans diverged from a common ancestor around 600 – 800 TYA, 6 most likely H. ergaster or heidelbergensis. 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.
Early humans 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, with Homo sapiens the sole human inheritor. This makes us very curious about which fossil human species were evolutionary dead-ends and which ones belonged to the lineage that still lives on with us.
Two important themes in human history are called multiregional evolution and the recent African evolution of modern humans. They are often presented as competing alternatives, though in fact they are both important aspects of the long human journey. Conquering diverse habitats across half the globe, Homo exhibits a new “multiregional” kind of worldwide evolution. Most mating takes place locally, but there are no true human-proof barriers between multiple mating regions. This broad but continuous habitat creates two opposing forces. Long-range isolation stimulates differences between regions, while gene flow irons out the differences from one region to another. Gene flow can occur when one group migrates and mates as it goes, or when communities mate in one gradual continuum across a continent.
The outcome depends on the balance between isolation and gene flow. If gene flow is too slow to keep up with isolation, populations on opposite ends of the continent will eventually diverge into multiple species. Dial up the gene flow fast enough, and you preserve the species as one unit. This species (whether it be across a continent or the globe) will have widespread commonalities but geographical variations that change with time.
Studies consistently show an ongoing pattern of human gene flow on the continental scale, confirmed for most of the last two million years. 7 There was a mass migration several hundred thousand years ago, which included the travels of Homo antecessor / heidelbergensis from Africa into Europe and Asia. 8 This mass migration was characterized by assimilation of new genes into old populations. 9 Neanderthal DNA is not completely extinct but still comprises a small percentage of today’s human genome. 10
We can’t reconstruct the exact balance between isolation and gene flow in all times and places throughout Homo history. Any combination of early humans could have contributed something to the modern human genome. However, holding together as a single global species for millions of years would be quite a stretch, and there is no compelling evidence that early humans did so. 11 Humans were spread so far-flung for so long that remote populations started to lose the capability to mate with each other. 12 Ultimately, Asian and European populations were overwhelmed by an exceptionally rapid mass migration of sapiens out of Africa, an adventure to be continued in Chapter 5. Since then, gene flow has been fast enough to keep humans evolving together as one species, multiregionally around the world.
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 the fairest 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. 14 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. 15 Likewise, if smarter early humans could outlive and outmate their dimwitted neighbors, they would have more egg headed children, initiating a cerebral arms race. 16
The underlying assumption here, though, is that larger brains are smarter. The brain size / intelligence correlation is actually pretty weak, especially among individuals within a species. 17 Compounding this, early humans did not display many immediate signs of intellectual progress. Aside from tools and fire, most indications of humanity’s remarkable intelligence occurred only within Chapter 5. It seems that the brain may have enlarged for different reasons, secondarily acquiring an exceptional potential that was exploited later.
Could it have had something to do with climate? The synchronization of the big brain bang with the ice ages, and with humanity’s occupation of new ecosystems, 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. 18
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. 19 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 a record breaker by any single metric. 20 Elephants and whales have larger brains with more neurons. Some birds have brains that are larger compared to their bodies. We seem to have gotten lucky with a double whammy: primate brain structure augmented by human brain size. Primates have unusually dense and fast neurons. 21 Then the big brain bang made human brains exceptionally large and complex 22 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. 23 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.
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. 24 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 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 carefully 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. 25 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 made the next great breakthrough, which archaeologists call Acheulean industry. The main tool in the Acheulean kit is called the bifacial hand axe. It is a core stone carefully carved into a hand-sized teardrop shape. We can only speculate about its use: Was it a handheld meat cleaver? Did humans throw it at animals? Was it used as a weapon against other humans? It was clearly important, as Acheulean industry eventually spread throughout most of the inhabited Old World. To our eyes, the most striking feature of the Acheulean hand axe is its symmetry. Even a child can tell at a glance that these axes were human made. Looking at one gives a glimpse into a mind undeniably capable of conscious deliberation, 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. 26 It seems that tools made man just as man made tools.
- Heidelberg photo by Ryan Somma from Occoquan, USA, CC BY-SA (https://creativecommons.org/licenses/by-sa/2.0), https://commons.wikimedia.org/wiki/File:Bodo_cranium.jpg (accessed, saved, and archived 2/03/20). ↩
- Thure E. Cerling et al., “Woody cover and hominin environments in the past 6 million years”, Nature 476:51-56 at 55 (8/4/2011), 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 108(26):10432-6 (6/28/2011), http://www.pnas.org/content/108/26/10432 (accessed and saved 2/18/2018). ↩
- Takeshi Nishimura et al., “Impaired Air Conditioning within the Nasal Cavity in Flat-Faced Homo”, PLOS Computational Biology 12(3): e1004807 (3/24/2016), https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1004807 (accessed and saved 11/09/19). ↩
- Matthias Meyer et al., “Nuclear DNA sequences from the Middle Pleistocene Sima de los Huesos hominins”, Nature 531:504-518 (3/24/2016), www.nature.com/articles/nature17405 (accessed 2/18/18, saved 11/03/19). ↩
- Meyer (2016), ibid. at 506. ↩
- Alan Templeton, “Haplotype Trees and Modern Human Origins”, Yearbook of Physical Anthropology 128(S41):33-59 (12/20/2005), https://onlinelibrary.wiley.com/doi/abs/10.1002/ajpa.20351 (saved 2/25/18, last accessed 11/09/19). ↩
- Peter Bellwood, “Chapter 3: Migrating Hominins and the Rise of Our Own Species”, First Migrants, Wiley-Blackwell (2013) pp. 36 – 70 at 37 (Figure 3.1) and 51. ↩
- 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). ↩
- Fred H. Smith, Anthony B. Falsetti, and Steven M. Donnelly, “Modern Human Origins”, Yearbook of Physical Anthropology 32(S10):35-68, esp. at 51-54 (1989), https://onlinelibrary.wiley.com/doi/abs/10.1002/ajpa.1330320504 (accessed and saved 3/03/2018). ↩
- For example, Neanderthals and modern humans were barely viable when they mated 50 – 100 TYA. See e.g. Fernando L. Mendez et al., “The Divergence of Neandertal and Modern Human Y Chromosomes”, American Journal of Human Genetics 98(4):728-734 (4/07/2016), https://www.cell.com/ajhg/fulltext/S0002-9297(16)30033-7 (accessed and saved 1/31/20). ↩
- Graph: Most data points taken from Chris Scarre, ed., The Human Past, Thames & Hudson (London, 2005), pp. 62 – 65 and 90 – 91. Graph by Scot Fagerland. ↩
- 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 proper weight H. sapiens. ↩
- Kwang Hyun Ko, “Origins of human intelligence: The chain of tool-making and brain evolution”, Anthropological Notebooks 22 (1):5-22 (Apr., 2016), http://www.drustvo-antropologov.si/AN/PDF/2016_1/Anthropological_Notebooks_XXII_1_Ko.pdf (accessed and saved 3/18/18). ↩
- The “social intelligence hypothesis” grew out of Alison Jolly’s research on primates in general. See e.g. Alison Jolly, “Lemur Social Behavior and Primate Intelligence”, Science 153(3735):501-506 (July, 1966), http://science.sciencemag.org/content/153/3735/501 (accessed and saved 3/18/18). ↩
- 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., 637 ff. at 650-651, G.J. Siegel editor, Lippincott-Raven Publishers (Philadelphia, 1999), https://fordham.bepress.com/chem_facultypubs/81/ (saved 3/18/18, last accessed 11/03/19). ↩
- 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 3/11/18). ↩
- Gerhard Roth and Ursula Dicke, “Evolution of the brain and intelligence”, Trends in Cognitive Sciences 9(5):250-257 (May, 2005), https://www.cell.com/trends/cognitive-sciences/fulltext/S1364-6613(05)00082-3 (saved 3/10/18, last accessed 11/03/19). ↩
- 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), pp. 12 – 32, https://www.oxfordhandbooks.com/view/10.1093/oxfordhb/9780195396690.001.0001/oxfordhb-9780195396690-e-002 (saved 5/26/18, last accessed 11/04/19). On p. 13, Flinn enumerates ways in which the human brain has grown more complex as well as large. ↩
- For example, see Julie J. Lesnik and J. Francis 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(9-10):354-356 (Sep – Oct 2007), http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S0038-23532007000500002 (saved 3/25/18, last accessed 11/04/19). ↩
- 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, last accessed 11/04/19). ↩
- 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, https://portal.research.lu.se/portal/en/publications/the-use-and-abuse-of-experimental-flintknapping-in-archaeology(ce0e4116-4593-4195-a69a-bfbd8aa7e0e7).html (saved 3/24/18, last accessed 11/04/19). 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 111(1):121-124 (1/07/2014), http://www.pnas.org/content/111/1/121 (accessed and saved 3/25/18, last accessed 11/04/19). ↩
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