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 strongly resembled modern humans 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. These early Homo species looked so much alike that it takes an expert to tell them apart. 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.
Conquering diverse habitats across half the globe, Homo exhibited a new multiregional kind of evolution. Most mating took place locally, but there were no true human-proof barriers between multiple mating regions. A broad but continuous habitat creates two opposing poles on an evolutionary spectrum. We can understand those two opposite pressures in terms of gene flow, or genetic inter-mixing between populations.
Imagine that the human species consisted of just two populations, one in Europe and the other in southeastern Asia, with absolutely no contact: Gene flow = 0%. These populations would adapt to completely different environments. Eventually, they would probably lose the capability to mate, becoming two species.
At the other extreme, imagine a “red” population and a “blue” population moving into the same small poplar forest and mating with each other at random: Gene flow = 100%. Before too long, their community would become one homogeneous hybrid of purple poplar people.
Multiregional evolution represents the middle-ground reality where humans are scattered far and wide but still interconnected. 2 You could imagine a whole world map with “hotspots” showing regions that have high gene flow with the poplar people and “cold spots” of low gene flow. If the map had many spots that remained persistently cold enough, chances are that humans would eventually diverge into multiple species. If those cold spots were just warm enough, though, humans would hold together as a single mating species. They would have widespread commonalities but geographical variations that change with time, such as we observe today.
One of today’s leading questions is whether the Homo genus ever reached that extreme point of speciation or if it has remained one loosely connected species ever since Homo erectus left Africa. Based on skeletal features, 10 – 20 different human forms have walked this earth. 7 Now there is undeniably only one surviving human species. For each ancient form, then, there are only two possibilities: either it was our ancestor, or it went extinct. Do any early humans represent evolutionary dead-ends? Or are we a hybrid blend of all of them? Skeletal differences alone do not give us that information; only DNA can tell the tale.
DNA evidence shows that gene flow throughout the Old World has been the rule, not the exception, for most of the last million years. 8 This intercontinental admixture dates back at least to a mass migration of Homo antecessor / heidelbergensis from Africa into Europe and Asia several hundred thousand years ago. 9 In fact, they mated with populations that had left Africa a million years before them. 10 This tangled tree shows that a broad swath of the Homo genus has remained, strictly speaking, one viable species all along. It is easier to confirm survivors than to rule out extinct branches. For instance, Neanderthal DNA did not go completely extinct but probably accounts for a small percentage of your own genome, unless you have deep roots in sub-Saharan Africa. 11
Realistically, we can’t reconstruct the exact map of gene flow and isolation throughout Homo history. Any combination of early humans could have contributed something to the modern gene pool, though we know that they did not do so in equal amounts. Every snippet of the human genome has its own history. Those that display diversity can be traced back to their most recent common ancestors. Many of those ancestors date back about a million years ago. They were heavily concentrated in Africa. A few were Asian or possibly European. 12
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, 3 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 4 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 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. ↩
- The forms named thus far include Homo antecessor, cepranensis, erectus, ergaster, floresiensis, gautengensis, georgicus, habilis, heidelbergensis, luzonensis, naledi, neanderthalensis, rhodesiensis, rudolfensis, sapiens, and tsaichangensis, as well as the Denisovans and the Red Deer Cave People. Wikipedia contributors, “Homo,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Homo&oldid=979218569 (accessed 9/20/20). ↩
- Alan Templeton, “Chapter 7: Human Population History over the Last Two Million Years”, Human Population Genetics and Genomics, Academic Press/Elsevier, Oxford (2019), esp. at 207 – 208. ↩
- 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). ↩
- Naoyuki Takahata, Sang-Hee Lee, and Yoko Satta, “Testing Multiregionality of Modern Human Origins”, Molecular Biology and Evolution 18(2):172-183 (2/01/2001), https://academic.oup.com/mbe/article/18/2/172/1079265 (accessed and saved 9/20/20). See esp. Table 2, p. 178. ↩
- 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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