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Are all humans related to all other humans?

  1. May 5, 2017 #1
    I read someone write "If you go back far enough, pretty much everyone is related to everyone." I wonder if all humans are cousins of all other humans. Perhaps the answer would depend on whether or not all humans alive today are directly descended from the first humans to evolve from lower primates. Let's use as an example the first human to evolve from lower primates. I will call the first human to evolve from lower primates Jane. And then I will call the lower primate closest to a human Joe. Joe was a lower primate closer to a human genetically than any lower primate that ever lived. Could Joe impregnate Jane if they had sex with each other? If not, how did the first two humans come about? Would the first two humans to have human offspring have to be two lower primates of opposite sex who had the same identical genetic mutations?

    Are all humans related genetically to all other humans?
     
  2. jcsd
  3. May 5, 2017 #2

    fresh_42

    Staff: Mentor

  4. May 5, 2017 #3

    BillTre

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    Science Advisor

    Basically, yes.

    Mitochondrial Eve and Y-Chromosomal Adam have been defined through molecular genetics as the most recent common ancestors that have been inherited in a CONTINUOUS female or male line. The mitochondrial genome is only transmitted from mothers to offspring, while the y-chromosome is only inherited through the male line from fathers (and is not even found in normal females). These mitochondrial and Y-chromosome genomes do not exchange genetic information with other mitochondrial and Y-chromosomal genomes because they do not have homologous unrelated chromosomes to exchange bits of DNA with. These are only samples of the inherited genome (mitochondrial DNA and Y-chromosome), there are a lot of other parts of the genome that are not so cooperative with researchers.
    The evolutionary history of the rest of the genome is therefore not so obvious.

    It should be noted however that even though particular parts of the genome can be traced back to hypothetical individuals, it does not mean that there were only those few people around. There are breeding populations passing genes (through breeding) back and forth among themselves. Its just that the descendants of particular mitochondrial or Y-chromosomal genomes were the ones passed on to current populations.

    Probably yes.
    As pointed out by @fresh_42, many humans carry Neanderthal and Denisovan DNA in our genomes which are though to be there due to successful matings between early humans and the presumed separate species, Neanderthals and Denisovans.

    Another point to make is that as time flows by, and the first "members of species" evolves, it is not necessarily obvious at the time. Normally, each little step in evolution will result in only small differences and not create instant incompatibilities between the old and new species. Reproductive barriers are not necessarily the defining trait of being a separate species and in some cases reproductive barriers are thought to arise later in the process, if they occur.
    It would be difficult to draw a line in a complex lineage of many individuals (see link above) where one side of the line would be species a and the other side would be species b. The animals above that line could be interpreted as isolated populations. And if you could draw a species dividing line, breeding would probably occur across that line anyway early in the separation of the two.
    As (lots of) time passes, the small changes add up to make the species separation more obvious in retrospect.
     
  5. May 5, 2017 #4
    Much of the human genome is similar to any other form of life on Earth.
    Some of our DNA is the same as plants.
     
  6. May 6, 2017 #5
    It is possible that human "extinction events" have lowered human genetic diversity.
     
  7. May 6, 2017 #6

    jim mcnamara

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    Staff: Mentor

    Very possible, especially in localized island populations.

    Population bottleneck (founder effect, what you are referring to, is one result):
    https://en.wikipedia.org/wiki/Population_bottleneck
    This has a link to founder effect in human populations. Both are good reads.

    You may encounter the Toba catastrophe theory - it postulated that 70000 ya there was mass extinction of humans due to the Toba eruption with concommitent reduction in genetic diversity. Subsequent DNA studies on humans assert that for about the past 100000 years there have not been events that reduced genetic diversity in humans. So this is a counterpoint to that hypothesis. I would hope that @Ygggdrasil knows a lot more about this.
     
  8. May 6, 2017 #7
    Is looks to be true that there was two waves of human migration frorm Africa to elsewhere.
    A popular story is that what we call Neadethals had happily occupied what now is the south of Europe..
    Then later the more aggressive Homo sapien' arrives,so the neanders get wiped out.
    Still. not really quite a different species though, I hear that many Europeans still have a few percent of Neander genes, including me.
    Other travelers invented China (Xian Wa) while passing through it.
     
    Last edited: May 6, 2017
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