Genetically, we’re all pretty much the same. A massive volcanic eruption 75,000 years ago may be why.

Lake Toba is all that is left of the volcano that nearly wiped out mankind.

Last blog I talked about how East Africans are genetically more diverse than Asians. Who are genetically more diverse than Native Americans.

From all of this you might have concluded that people are pretty different from each other. They aren’t.

People are surprisingly similar at a genetic level. For example, any two people from anywhere on Earth are more similar than two chimps from the same troop. Why are we all so alike?

One possible explanation is that something in our collective past nearly wiped us all out. And we all come from the few survivors who were left.

A likely candidate for this near annihilation event is the Toba volcanic eruption that happened in Indonesia 75,000 or so years ago. This eruption was huge.

It was equivalent to around 1 billion tons of dynamite and was about 3000 times more powerful than the Mount Saint Helens eruption in 1980. It also may have reduced the average global temperature by 5 degrees Celsius, darkened the world for 5 or 6 years, and plunged the world into a new Ice Age.

As you might imagine, this eruption had dramatic effects on species around the world including our own. Estimates of how many people were left range from around 1000-10,000 breeding pairs. The theory is that we are all so alike because we share these survivors’ DNA.

Whether true or not, a bottleneck in our past would not make us unique. Lots of species go through these near death experiences.

Scientists think cheetahs went through one around 10,000 years ago. Cheetahs are all so similar genetically that veterinarians can do skin grafts with “unrelated” cheetahs.

And of course, people have created bottlenecks in species too. For example, in the late 1890’s there may have only been 20-100 elephant seals left in the world because of hunting. Now there are at least 150,000 spread across the west coast.

Species are in danger long after they go through a bottleneck. They have a pretty limited gene pool which means they may not be particularly healthy and are in danger of being wiped out by, for example, a single disease. Humans are probably OK in this regard (consider natural resistance to HIV for example) but elephant seals, bison, and cheetahs, and many other species may not be.

Fortunately for us we successfully came through our bottleneck. Hopefully, the animals that we’ve nearly wiped out will too.

Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA.

Explosive hypothesis about humans’ lack of genetic diversity 10 November,2016Dr. Barry Starr
  • Pingback: Hug-a-helix: celebrate DNA Day, April 25th | QUEST Community Science Blog - KQED()

  • Pingback: And Then There Were 44 | QUEST Community Science Blog - KQED()

  • Terry Davis

    I think humans are pretty diverse. Chimps don’t have different skin, hair and eye color. We’re not as diverse as dogs, though. Does that mean we’ve undergone lots of harsh evolution in a short time?

    I really don’t care if human DNA is 99.99999% the same. What’s the point if we are clearly very different.

    • Mickelodian

      Thats not what they mean by diversity… diversity ‘within’ the human species is narrow than ‘within’ on troop of chimps… you can’t tell if one chimp looks markedly dissimilar to another, you are not a chimp! You are used to discerning the differences in humans, you are excellent at that… but that because you are the same species as all those folks… so you are able to tell the difference.

      In onje chimp troop, to the chimps it really does look like a mish mash of the UN council! To them…

      In terms of genetics the diversity of humans is very narrow considering the population size… That does not mean its 99.9999% …even twins don’t have that level of similarity… its a diversity range of about 99.5% to 99.6% for humans.

  • Víktor BiR

    Learn about genotype and fenotype, please.

  • Luk

    thanks, intresting article

  • Khuram

    How this theory is different from Naoh Ark’s…???

    And for Cheetahs:

    But many insects like flies, ants also all look almost same…??? So what is their difference with Cheetahs…???

    And despite genetic similarity in humans … humans seem most diverse in terms of facial and other body appearances. What’s the reason…???

    • Barry

      What makes it different from Noah’s Ark is that this genetic similarity is not seen in all species. If there were an event in which every species was reduced to a single mating pair, then any two chimps should be as similar to each other as any two people. But they’re not…two people have much more similar DNA than do two chimpanzees. This tells us there was a human specific event that nearly wiped humans out that did not have a similar impact on chimpanzees. Cheetahs are the same everywhere…ants are very different genetically the world over.

      • Khuram

        Thankyou for the clarifications.

      • Wallace Barbosa de Souza

        “Of every clean beast thou shalt take to thee by sevens, the male and his female: and of beasts that are not clean by two, the male and his female.
        Of fowls also of the air by sevens, the male and the female; to keep seed alive upon the face of all the earth.”

        Well, of many species (the “clean” ones) has been taken 7 couples each 🙂
        Also, most animals reproduce much more fast that human does… Not to mention polyploidy, hybridization, etc..
        And finally, from the 8 persons at ark, there were 3 brothers, who married 3 women, it means, from the 3 couples we already have 50% of genetic equality, due to them (Noah’s sons)… Surely a very very narrow bottleneck!

        • Mickelodian

          Insisting that 8 folks on a boat is the answer… for fucks sake what year is it? 1750 or something?

      • Mickelodian

        This event affected ALL of the hominids…. it also resulted in the separation fo the lowland and highland gorilla in terms of dating.

        BUT it does seem to have almost wiped out humans… or rather humans almost became extinct at the same time. There is a correlation, but no evidence of causality.

        I would be more inclined to go along with a ‘human specific’ cause. But heres the thing…we do have other species of closely related hominid species to derive more information… Did the neanderthals also bottleneck, they were in Europe at the time. How about the Denisova? Or other hominids? How about PRE-toba… was our DNA as low in diversity before this event… if so then it might not have helped but it was not the root cause.

        When we have enough samples to fully check their diversity and compare to ours at the time we’ll have a better idea.

  • Pingback: Genius bonobo chimp creates stone tools like early hominids - Page 9()

  • dovhenis

    Update NOW with OLD essay:

    Seed of
    Human-Chimp Genomes Diversity

    2 Nov,2005 Dov,
    in biologicalEvolution forum.

    Evolution’s Seeds of Diversity, Human and Chimpanzee/Bonobo Genomes.

    Chapter One, In which some wonder what made us human.

    Three recent
    quotations from Science, representative of many other recent similar statements
    in various scientific publications:

    A) “Understanding
    the genetic basis of how genotype generates phenotype will require increasing
    the accuracy and completeness of the currently available chimpanzee genome
    sequence, as well as sequencing other primate genomes.”

    B)”Can we now
    provide a DNA-based answer to the fascinating and fundamental question, “What
    makes us human?” Not at all! Comparison of the human and chimpanzee genomes has
    not yet offered any major insights into the genetic elements that underlie
    bipedal locomotion, big brain, linguistic abilities, elaborated abstract
    thought, or any other unique aspect of the human phenome.”

    C)”What makes us
    human? This question may be answered by comparison of human and chimpanzee
    genomes and phenomes, and ultimately those of other primates. To this end, we
    need to understand how genotype generates phenotype, and how this process is
    influenced by the physical, biological, and cultural environment.”

    Chapter Two, In which is explained plainly and
    succinctly the obvious route by which we evolved, i.e. that genotype has not
    generated phenotype, that we evolved from our genotype via a group of feedback

    From Science, Vol
    308, Issue 5728, 1563-1565 , 10 June 2005, Immunology: Opposites Attract in
    Differentiating T Cells, Mark Bix, Sunhwa Kim,Anjana Rao: “During
    differentiation, precursor cells with progressively narrowed potential give
    rise to progeny cells that adopt one of two (or more) divergent cell fates.
    This choice is influenced by intricate regulatory networks acting at multiple
    levels. Early in differentiation, precursor cells show low-level activation of
    all progeny genetic programs. Bias toward a given lineage comes from
    environmental inputs that activate powerful positive- and negative- feedback
    loops, which work in concert to impose selective gene expression patterns”.

    Chapter Three, In which we explain the revolutionary
    evolved uniqueness of the human ape’s phenotype: The 6My-old revolutionary life
    evolution was initiated by our forefathers who adapted from life in semi- or
    tropical forest circumstances to life on plains. Changes in living posture and
    circumstances led to modified perceptive/adaptive experiences and capabilities.
    Developing employment of tools effected enhanced differentiation of hands from
    legs and enhanced upstanding posturing. As evolving community culture led to
    language communication humans have gradually replaced adaptation to changed
    circumstances with self-evolving cultures/civilizations for control and
    modification of much of their circumstances. This is essentially similar to
    early life’s celling evolution, but with culture functioning for humans for
    change/control of circumstances in lieu of genetic and protein toolings that
    function for the in-cell genomes for adapting their cell’s physiology to
    changing circumstances.

    Chapter Four, In which appears, may be, genetic
    evidence/demonstration of the workings of human cultural evolution.

    (a) From Science,
    2 Sept 2005: “Page’s team compared human and chimp Ys to see whether either
    lineage has lost functional genes since they split. The researchers found that
    the chimp had indeed suffered the slings and arrows of evolutionary fortune. Of
    the 16 functional genes in this part of the human Y, chimps had lost the
    function of five due to mutations. In contrast, humans had all 11 functional
    genes also seen on the chimp Y. “The human Y chromosome hasn’t lost a gene in 6
    million years,” says Page. “It seems like the demise of the hypothesis of the
    demise of the Y,” says geneticist Andrew Clark of Cornell University in Ithaca,
    New York.”

    (b) But look at
    this: From Science, Vol 309, 16 Sept 2005, Evolving Sequence and Expression:”An
    analysis of the evolution of both gene sequences and expression patterns in
    humans and chimpanzees…shows that…surprisingly, genes expressed in the brain
    have changed more on the human lineage than on the chimpanzee lineage, not only
    in terms of gene expression but also in terms of amino acid sequences”.


    Chapter Five and conclusion,

    In which I
    suggest that detailed study of other creatures that, like humans, underwent
    radical change of living circumstances, for example ocean-dwelling mammals,
    might bring to light unique evolutionary processes and features of evolutionary
    implications similar to those of humans.


    Dov Henis

    (comments from
    22nd century)

    Earth life
    genesis from aromaticity-H bonding


  • Chris Roat

    What is it about chimps that allows them to be so genetically diverse within a troop, given that they would have gone through the same genetic bottleneck during the Toba eruption?

    • Wallace B…

      Toba eruption is a myth, man..

  • Pingback: The rule of law or not - Page 2()

  • Avery Wang

    This doesn’t make sense. Why would a volcanic eruption in Indonesia cause a bottleneck event selectively for humans? If it were due to cooling then it should affected many more species,

    • Mickelodian

      “If it were due to cooling then it should affected many more species,”

      It did…. at the same time all mammals that weighed in above a certain level became went through a genetic bottleneck event… or indeed died out. Any event that impacts badly the environment, specifically the plant life.. will have a huge effect on large fauna.

      ALL of the hominids bottle necked, (as far as we know) clearly we all utilized something that was damaged in this event. It does seem to have been particularly harsh on the hominids… might be that the forests dissipated quickly leaving only isolated pockets which slowly died out… birds can fly away and small mammals breed fast and are a lot more diverse.

      Among the ape family humans are unique in that there is less diversity in the whole of humanity than there is diversity in one troop of chimpanzees. That is obviously NOT compatible with this bottleneck event. However if several distinct troops of other apes closeted together afterward but we could not then it would explain why their numbers reduced but their diversity is not as low as ours. It would also explain why we had no choice but to adapt or die out. The odds were seriously not in our favor there.

      The answer I suppose is we don’t know exactly what the result of this mega eruption was or the impact it had on our ancestors… but that unknown will not stay unknown for very much longer. Like all scientific questions its not an ‘IF’ it will be explained its more of a ‘WHEN’ it will be explained.


Dr. Barry Starr

Dr. Barry Starr (@geneticsboy) is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA and runs their Stanford at The Tech program. The program is part of an ongoing collaboration between the Stanford Department of Genetics and The Tech Museum of Innovation. Together these two partners created the Genetics: Technology with a Twist exhibition.

You can also see additional posts by Barry at KQED Science, and read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.

Sponsored by

Become a KQED sponsor