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Dating the Origin of Us

Theoretical anthropogeny seeks to understand how Homo sapiens rose to a position of global dominance.

By | November 1, 2013

LAST KNOWN SURVIVORS: Most clades of mammals consist of multiple, potentially cross-fertile species, each occupying its own ecological niche. We humans are an unusual exception, having replaced every related species wherever we went, with minimal interbreeding (black arrows).FIGURE BY JESSIE ROBIE AND PASCAL GAGNEUX

When did “behaviorally modern humans” (BMHs)—i.e., us, the single species that has taken over the planet using our unusual cognitive abilities—first arise? We do not know for sure, but anthropogeny, which takes a systematic approach to explaining human origins, offers some clues. Advances in anthropogeny require a broad array of information gathered from numerous disciplines. But most human-origin events are shrouded in mystery and difficult to reconstruct, making this a discipline where setting up thought experiments could be useful.

Let’s first ask if the cognitive abilities of BMHs are strictly genetically wired, or derived from gene-environment interactions. Imagine that we place 1,000 healthy, unrelated newborn babies on a predator-free deserted island, where silent, legless robots tend optimally to their bodily needs. If the robots leave after 15 years, what would we find 30 years later? I have posed this thought experiment to learned individuals from varied backgrounds and received widely divergent answers. At one extreme, some ask if these humans would even be standing upright, given the absence of adult role models and encouragement. At the other extreme, some predict that they would have already invented a complex language and some tools. Even assuming the best-case scenario, I believe these humans would be less accomplished than our ancestors at the time of BMH origins, having the same genetically-wired cognitive potential, but very limited expression of this potential. So, it is obvious that behavioral modernity is not fully genetically encoded; we needed ongoing interactions with others like us over hundreds of generations to bootstrap culture and reach our current status. This raises the question: When and where did BMHs originate?

There used to be two competing theories about this origin. The “multiregional hypothesis” posited that different “races” of humans evolved in parallel from more ancient Homo erectus ancestors who had spread out from Africa starting ~2 million years ago. The alternate view, better supported by available facts, was the “Out of Africa” theory, which posited that all humans alive today came from a single origin in Africa, involving perhaps fewer than 10,000 individuals. Today, a hybrid theory posits an initial Out of Africa event, but includes a small degree of interbreeding between BMHs and other extant cousins, such as Neanderthals, Denisovans, and unknown African lineages. While researchers continue to work out the details of this limited interbreeding, other unanswered questions remain.

Evidence for “anatomically modern humans” (AMHs, individuals whose fossil skeletons look much like ours), goes back ~200,000 years in Africa. But AMHs were not necessarily BMHs. Indeed, the archaeological finds surrounding older AMHs in Africa show no evidence of some types of activities (such as personal ornamentation with beads, or burials accompanied by funerary items) that one might associate with current human “mentalizing abilities” (so-called “theory of mind,” or the ability to fully attribute mental states to others, similar to one’s own). Thus, it remains unknown whether the African AMHs of 100–200 thousand years ago had the same cognitive potential as humans today. Or do our fully modern cognitive capacities go back even deeper in time, to our common ancestor with Denisovans and Neanderthals ~500,000 years ago (Homo heidelbergensis)? Or even earlier, to ~1–2 million years ago (Homo erectus)?

For the moment, we are limited in what the data can tell us, so it is worth trying more thought experiments. If we took 1,000 newborn BMH babies from 60–70,000 years ago in Africa, brought them to the United States today, and gave them every type of support and opportunity, would they end up being representative of the current human population? Very likely, yes. In some sense, this experiment has already been done. People have migrated to the U.S. from all over the world, and if their children are given every opportunity, they generally exhibit much the same behaviors and abilities (just visit students on the UC San Diego campus). Now consider what would happen if we did the same experiment with 1,000 newborn AMH babies from Africa ~100–200,000 years ago. How would they fare? The answer is far less clear.

At a May 2013 symposium organized by the UC San Diego/Salk Institute Center for Academic Research and Training in Anthropogeny (CARTA), international experts gave talks on subjects related to the origin of BMHs. Follow-up discussions among faculty and graduate students in our anthropogeny-specialization track indicated a wide range of opinions regarding when the “behaviorally modern” cognitive capacities of humans originated. I therefore presented the aforementioned thought experiment about transporting 1,000 healthy newborn babies from older times into today’s environment—asking how far back in time one could go and still elicit a full BMH cognitive phenotype.

I personally favor ~100,000 years ago, for several reasons. First, most current genetic data point to a common origin of current humans at about that time. Second, the humans who entered the Eastern Mediterranean (the Levant) at that time were burying their dead with funerary items, and transporting perforated shell beads (presumably made into necklaces) over rather long distances. Third, my group’s work suggests a population bottleneck caused by infectious disease just before that time. Finally, despite evidence for cross-fertility with other ancient hominins (including some in Africa), we BMHs remained largely genetically distinct, despite tens of thousands of years of opportunity to mate and mingle into a distinct hybrid species in each locale. While this outcome could have been due to chance, I think it more likely that the initial hybrids were lacking a fully human-like theory of mind, and that the “cognitively sterile” hybrids could not easily incorporate themselves (or their genes) back into the BMH population.

Interestingly, the answers to an anonymous poll of the same faculty and students varied widely among the five possible time periods and populations I suggested (ranging from ~60,000 to ~500,000 years ago), with some respondents including Neanderthals as being equivalent to BMHs. So, even among experts and bright students who had just heard the latest research reports, there was no consensus on this issue. Much more data from many disciplines are required before there is any hope of finding the answer.

A related question is whether there was something cognitively special about BMH minds that allowed us to systematically replace all the other intelligent hominins we encountered as we moved around and out of Africa. And did those cognitively special attributes evolve gradually via typical natural selection—or was there a psychological evolutionary barrier that we BMHs breached on our way to global domination? 

Ajit Varki is Distinguished Professor of Medicine and Cellular Medicine at the University of California, San Diego, codirector of CARTA, and coauthor with the late Danny Brower of the book Denial: Self-Deception, False Beliefs, and the Origins of the Human Mind, which discusses these questions in more depth. Videos of CARTA symposium talks can be found here.

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Avatar of: James V. Kohl

James V. Kohl

Posts: 171

November 4, 2013

Re: "Evidence for “anatomically modern humans” (AMHs, individuals whose fossil skeletons look much like ours), goes back ~200,000 years in Africa."

The following publication takes the ~200,000 years back to ~2,000,000 years in Eurasia and Africa via "normal variations" that were interpreted as evidence of speciation. Are we now simply going to ignore that published work?

A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo

Avatar of: RReis

RReis

Posts: 1

November 4, 2013

Wonderful article.  This is not my field but I love reading about it and I think the idea of these thought experiments is great.

Do you know if there has been any examples of infants from hunter-gatherer societies who, for whatever reason, were removed from that society and raised in a highly developed society like the IUS., and if so what was the result? Of course it would be even better if the adopting parents didn't know the orgin of their new baby.

I'm sure this is the case at least for a few infants but again this is not my field,  

Thanks again

Avatar of: mightythor

mightythor

Posts: 43

November 18, 2013

A nice picture is beginning to emerge of successive waves of hominins migrating out of Africa into the Eurasian continent.  And yet, Africa remained the crucible of human evolution, despite the wide distribution of hominins over the highly diverse geography of Eurasia.  The explanation may be trivial: disease. Hominins weren't the only African emigrants. Each successive wave of hominins must have carried with it novel pathogens, derived from contact with other primates in Africa, to which resident Eurasian populations had no resistance. The newcomers weren't necessarily better, but they had nastier bugs. All kinds of interesting indigenous Eurasian hominin evolutionary adaptations may have been lost this way. This "African advantage" persisted until domesticated animals became the primary source of novel human pathogens.  In scrutinizing the anatomy and behavior of H. sapiens for clues to our evolutionary dominance, we may be looking in the wrong direction.

Avatar of: Berwal

Berwal

Posts: 2

November 18, 2013

The 1000 infants with robots, irrespective of from which era they came,  would likely perish long before the robots left.  In the absence of human or other mammalian source of co-regulation (see Porges, Polyvagal Theory) they could not develop the brain structures associated with normal human social interactions.  Likely even the capacity for language development would also be impeded.

While the intent of your experimental set-up is not perhaps about these shortcomings, it seems that evolutionary biology would do well to consider human species development with a wider lens.  Is the Wall Street banker or the Ivy League academic more evolved than an isolated Amazonian aboriginal? What are the characteristics that lead to success under these widely different situations?

Avatar of: James V. Kohl

James V. Kohl

Posts: 171

November 18, 2013

Analysis of 6,515 exomes reveals the recent origin of most human protein-coding variants suggests that the majority of changes that make us 'modern humans' have occurred during the past 10,000 to 5,000 years. This seems to be consistent with evidence of mouse to human differences attributed to a nutrient-dependent amino acid substitution and changes in teeth, mammary tissue, skin glands, and hair thickness in a human population in what is now central China that appears to have adapted the the environment during the past 30K years.

This is not consistent with any theory of mutation-initiated natural selection. Instead, it is a refutation of mutation-driven evolution that can be addressed in the context of nutrient-dependent pheromone-controlled adaptive evolution in species from microbes to man via the conserved molecular mechanisms of ecological, social, neurogenic, and socio-cognitive niche construction. 

How else but via nutrient uptake do you get from 20-30,000 genes to 200,000 to 300,000 different proteins?  How else but via the pheromone-controlled physiology of reproduction do you get to species diversity.

Avatar of: kpetrak

kpetrak

Posts: 9

November 18, 2013

Maybe “behaviorally modern humans” (BMHs) came from outer space... "Scientific speculations" is such a nice passtime!

Avatar of: Hugh-F-61

Hugh-F-61

Posts: 39

Replied to a comment from James V. Kohl made on November 4, 2013

November 18, 2013

They were not "Anatomically Modern", they had relatively small brains and projecting faces. I think the paper is suggesting that Homo "species" in Africa and Eurasia had a similar range of skull size/shapes 1,800,000 years ago, and that at that time they were one continuous population.

Avatar of: Sean McHugh

Sean McHugh

Posts: 1

November 19, 2013

"what would happen if we did the same experiment with 1,000 newborn AMH babies from Africa ~100–200,000 years ago. How would they fare? The answer is far less clear."

Gosh! Is it?

I don't think I have read a piece less scientific than this one in your magazine.

I recall Stephen Colbert coining the term 'Truthiness' for a similar appeal to gut feelings during the Bush regime in the USA.

Perhaps this is the first appearance of Scienciness, where gut feelings, thought experiments and technical jargon take the place of saying the obvious: 'We don't know. We will never know.'

Avatar of: rb336

rb336

Posts: 1

November 19, 2013

The DNA and bones agree - Neanderthal and "anatomically modern" humans interbred.  Neanderthal should be H sapiens neanderthalensis

Avatar of: Alexandru

Alexandru

Posts: 70

November 19, 2013

Please, do not laugh and go back to creationism bill (http://the-scientist.com/2012/02/02/indiana-senate-backs-creationism-bill/), and listen the first human evolution description in the world:

"Winds were blowing from all directions and lashing the surface of the ocean. Four huge beasts came up out of the ocean, each one different from the others. The first one looked like a lion, but had wings like an eagle. While I was watching, the wings were torn off. The beast was lifted up and made to stand like a man. And then a human mind was given to it." (Daniel 7.2-4)

What that means winds in that context?

In my opinion, wind is a high frequency magnetic wave transporting the God information to the earth, because “the seed is the word of God” (Luke 8.11)

Because I "do not conform to the standards of this world, and let God transform me inwardly by a complete change of my mind" (Paul, Romans 12.2), “I honor God for what He conceals; and I start to explain!” (Solomon, Proverbs 25.2) that the spermatozoid is the microphone of God.

Despite the geneticists speculation that only Eve mtDNA is inherited, please, read my scientific theoretically demonstration of paternal mtDNA inheritance, that complete Eve mtDNA theory: Mitochondrial Adam DNA data transmission theory (ISBN 978-606-92107-1-0), based on magnetic field communication theory.

"The first man, Adam, was created a living being; but the last Adam is the life-giving spirit. It is not the spiritual that comes first, but the physical, and then the spiritual.The first Adam, made of earth, came from the earth; the second Adam came from heaven." (Paul, 1.Corinthians, 15.45-47)

"It is clear that Christ himself wrote this letter and sent it by us. It is written, not with ink but with the Spirit of the living God, and not on stone tablets but on human hearts." (Paul, 2 Corinthians 3.3)

"The word of God is alive and active, sharper than any double-edged sword. It cuts all the way through, to where soul and spirit meet, to where joints and marrow come together. It judges the desires and thoughts of man's heart. There is nothing that can be hidden from God; everything in all creation is exposed and lies open before his eyes. And it is to him that we must all give an account of ourselves." (Paul, Hebrew 4.12)

I patiently waiting that the scientists find in the xiphoid process the divine couple Adam mtDNA and Eve mtDNA.

…………

As Winston Churchill once said about courage, "I stood up and expressed my point of view and now I sit down and listen to the opinions of others involved in this dialogue."

Avatar of: RobB

RobB

Posts: 1

Replied to a comment from Alexandru made on November 19, 2013

November 24, 2013

Please seek help. You have no idea how mad your writings are.

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