5.III: Anatomically And Genetically Modern Humans

mitochondrial DNA haplogroups, Y chromosome haplogroups, phylogeography, human migration
These 3D maps show how our all-maternal (above) and all-paternal (below) ancestors have migrated through the world since the beginning of genetically modern humanity.  The oldest traceable individuals are labelled as the “L” and “Y” in squares.

A.  The Modern Skeleton
B.  Haplogroups And Migration Patterns
C.  Citations

A. The Modern Skeleton

The first fossil ever recognized as an early human was “Neanderthal 1”, discovered in 1856.  Paleontologists immediately knew that Neanderthal Man didn’t look “like us” – but what did that mean?  Two more centuries of fossil discoveries have forced scientists to come up with a definition of what “we” look like in order to track our species’ evolution.  There are just a few key features that set Homo sapiens apart from our extinct human predecessors.  The most important of these features are a rounded skull, a recessed face, a chin, and an overall more lightweight skeleton, especially visible in the brow ridge.

Scientists almost universally use the word “globular” to describe the sapiens skull.  Early humans had longer, flatter skulls that were somewhat pointed in the back.  It would be fair to say that they resembled American footballs compared to our soccer-football heads.  The modern forehead is nearly vertical, and the modern human face is set back further in alignment with the forehead.  The chin is probably just a byproduct of the changing jaw, but it is a unique H. sapiens marker.

Early humans, anatomically modern humans, Homo heidelbergensis
Early human, left; Anatomically Modern Human (AMH), right. A) AMH skull more globular. B) Sloping vs. vertical forehead. C) Browridge: heavy in early human, virtually gone in AMH. D) AMH lower face more “tucked in” under braincase. E) AMH chin juts forward, F) Smaller jaw and teeth in AMH reflect general gracilization. 1

These differences between heidelbergensis and sapiens might seem like minor examples of random shape shifting.  In fact, closer analysis reveals a consistent tradeoff: a larger braincase at the expense of chewing power. 2 For instance, the depressions on the outside of the skull, which accommodate the major chewing muscles, have become smaller in sapiens.  Our teeth and jaws are also smaller, as is our brow ridge, which offers structural support against the strains of chewing. 3 Of course, we don’t need vice-like jaws when we cook our food and process it with tools. 

These modern features did not evolve simultaneously, but also were not completely independent of each other.  300 TYO fossils in Morocco display modernizing faces with a still-archaic braincase. 4 The rounded skull did not appear until after 200 TYA, 5 and the chin within the last 100,000 years.  On the other hand, the skull is a complex 3D puzzle with 22 bones, and it is difficult to change one without modifying the others. 6 Just a few small independent changes could have led to the retracted face, globular braincase, reduced brow ridge, and even the shape of the vocal tract. 7  

The most recent change to the human skeleton – still an ongoing trend – is gracilization.  Human bones, in the body as well as the skull, are getting thinner and less dense.  Note that a thinner skullcap helps provide an even roomier braincase.  In the long bones, gracilization might be an environmental condition as much as a genetic one.  As living tissue, bones develop in response to exercise, just like muscles do.  Thinner bones can reflect a less active lifestyle.  In fact, gracilization has been most pronounced in the last 10,000 years as humans have made the transition from foraging to farming and settled life, 8 and is still accelerating with industrialization. 9 Gracilization has now reached pathological levels; as we get old, we are vulnerable to osteoporosis, deterioration of the bones to the point of brittleness.

B. Haplogroups And Migration Patterns

1.  Understanding Phylogeography
2.  The Global Family Tree

1. Understanding Phylogeography

Phylogeography studies the ancestry of modern human populations.  DNA analysis has helped reconstruct the major branches of the global family tree.  Before discussing the conclusions, it is necessary to understand a few key concepts about this field. 1

Phylogeography works by examining snippets 2 of DNA.  In prehistory, we must think of our ancestors not as individuals but as populations of interbreeding people.  A population is defined by its gene pool, the set of snippets that flows through the population.  Slow changes in the gene pool – the births and deaths of DNA snippets over hundreds of generations – leave records of our lineage.     

A genetic mutation in one person’s genes creates a new snippet.  If successful, the snippet will multiply throughout that person’s local gene pool.  To the extent that the population is isolated from others, that snippet will not escape into other populations.  This is how snippets serve as genetic markers to distinguish populations that have been isolated from each other for millennia.  When a population splits in two (e.g. some members emigrate), each subpopulation retains the original marker and then accumulates new ones of its own.  By studying patterns among living DNA samples from throughout the world, we can now retrace the travels of ancient ancestral populations.   

Even DNA snippets do not live forever.  If a population becomes critically small, some snippets will die out, and the survivors will become more prominent in the gene pool.  Geneticists call this situation a population bottleneck.  As phylogeographers, we have mixed feelings about bottlenecks.  They are limits to discovery; we cannot study the evolution of earlier extinct snippets.  On the other hand, they help us zero in on our ancestors.  Each single surviving snippet had its origin in one person, so genetic ancestry ultimately maps our descent from these individuals.  We can call them the first “genetically modern” humans.  All we can really know about them is approximately where and when they lived.            

Researchers focus most of their attention on mitochondrial DNA (mtDNA) and the Y chromosome.  These are the only snippets that do not intermix during sex but are copied in full for the next generation.  Your mtDNA comes from your all-female line of ancestors (your mother, her mother, etc.)  If you are male, your Y chromosome comes from your all-male line (your father, his father, etc.)  Each of these snippets contains the full record of its mutations since the last bottleneck.  The major world populations that have separated since the last mtDNA and Y chromosome bottlenecks are called haplogroups.  They are usually designated with capital letters, such as Y-haplogroup A. 

2. The Global Family Tree

When phylogeographers map out a family tree of human mtDNA throughout the world, they find that everyone’s all-maternal ancestry converges on one woman who lived in Africa 10 100 – 200 TYA. 11 She is nicknamed “Mitochondrial Eve”, an endearing but unfortunately confusing term.  She was an ordinary woman in her time. By a twist of fate, she just happens to be the last woman whose daughters’ daughters’ … daughters include all living women.  Everyone’s all-paternal lineages likewise coalesce in a “Y-Chromosomal Adam” 12 who, despite his name, never knew Mitochondrial Eve. 3 Current research places him in central – northwestern Africa 13 about 250,000 years ago 14 .    

These are the two most venerated modern human ancestors, though they only represent two modest portions of our DNA.  We have inherited the rest of our diversity from tens of thousands of “Adam and Eve” individuals 15 : the first genetically modern humans. As we already saw in Chapter 6, many of these ancestors lived a million years ago or more, and they were not all African. Taken together, their personal mutations form the basis of the 0.1% of the human genome that varies widely.   The other 99.9% of the genome, identical in everyone alive today, has persisted unchanged since before the first genetically modern humans.

Mitochondrial Eve, phylogeography, human genome project
Graphic showing how one lucky mother (black) came to be Mitochondrial Eve during a population bottleneck. All other lines (colored) died out when those women failed to have daughters. 16

On our mothers’ side, the great migration out of Africa is reflected in this simplified mitochondrial family tree:

Mitochondrial haplogroup L is the world’s oldest.  It is common throughout Africa and nowhere else.  Group L3, a subgroup of L, spawned groups M and N right around the time that Africans entered Asia.  The M and N branches of the family then spread from Asia to the rest of the world. 

A similar tree is exhibited on our fathers’ side.  The six major lines descending from Y-Chromosomal Adam are labeled haplogroups A – F.  The two oldest, A and B, are native to Africa.  The largest branch, Y-haplogroup F, originated in India about 50 TYA. 17 This group now encompasses 90% of the men outside of Africa. 

Oddly, the oldest line outside of Africa, Y-haplogroup C, is found in Australia!  Humans reached Australia as early as 65 TYA 18 and occupied the whole continent by 50 TYA.  They obviously traversed Asia to get there, but so far scientists have found only faint traces of their so-called “southern route”.  These were the first humans to reach Australia.  Interestingly, many large Australian animals went extinct around the same time.  It seems that human activity was a major factor. 19  

The settlement of Europe came surprisingly late.  Haplogroups originating from the Levant to the -stan countries spread westward into Europe around 40 – 45 TYA.  By 20 TYA, modern humans had reached the furthest shores of the Old World.

Two recent genetic findings are consequential.  First, no haplogroup is completely isolated from the others.  They are all intermixed, with regional hotspots for each one.  This spectrum pattern reminds us that the modern human species has evolved multiregionally since leaving Africa; we are one global family.  Related to this, the genetic variation from one geographic region to another is minor.  We are visually impressed by the physical features distinguishing, say, west Africans from east Asians.  These are due to distinct 20 , but slight 21 , genetic differences.   

Genetic testing services can reveal your mtDNA and Y-chromosome haplogroups. 22 Their reports are accompanied by maps and timelines.  It’s a fascinating way to connect your lifeline to the earliest Homo sapiens.    

Up to Chapter 5 

Back to Section 5.II:  Hello, Modern Humans; Goodbye, Early Humans

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Continue to Section 5.IV:  Behaviorally Modern Humans

C. Citations

  1. Left figure: Editor at large, CC0 license, Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Antecessor_Skull.png (dashed line filled in by Scot Fagerland).                      
    Right figure: By Ichthyovenator, CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0), from Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Sapiens_Skull.png (both accessed and saved 9/09/18, archived 12/31/19).
  2. David W. Cameron, “Appendix: Detailed Description of Characters”, Bones, Stones, and Molecules (with Colin P. Groves), Elsevier Academic Press (2004), pp. 287 – 344.  Cameron lists 19 metrics in which sapiens differs from ergaster, and five of them give sapiens a disadvantage in chewing strength.
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  12. Robert L. Dorit, Hiroki Akashi, and Walter Gilbert, “Absence of polymorphism at the ZFY locus on the human Y chromosome”, Science 268(5214):1183-1185 (5/26/1995), http://science.sciencemag.org/content/268/5214/1183/tab-pdf (accessed and saved 9/23/18).
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  16. Population bottleneck image by C. Rottensteiner (TiGen), CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons, https://upload.wikimedia.org/wikipedia/commons/9/92/MtDNA-MRCA-generations-Evolution.svg (accessed and saved 9/22/18, archived 12/31/19).
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  21. Richard Lewontin, “The Apportionment of Human Diversity”. In: T. Dobzhansky, M.K. Hecht, W.C. Steere (eds) Evolutionary Biology, Springer, 381-397 (New York, 1972), https://link.springer.com/chapter/10.1007%2F978-1-4684-9063-3_14 (accessed and saved 9/30/18).
  22. The two best tests for identifying your Y and mtDNA haplogroups are www.23AndMe.com and www.FamilyTreeDNA.com .  23AndMe also includes a Neanderthal report.  Both services provide autosomal analysis for your pre-colonial ancestral-region pie chart.  23AndMe is better for genetic health screening, while FamilyTreeDNA is preferred by genealogists (to locate distant cousins).  National Geographic’s Genographic Project is slightly more limited, but it is the authoritative resource for phylogeographical researchers.
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