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I have heard (from multiple sources) that the current scientific opinion is that the human species arose in Africa. What are the reasons for this opinion? If possible, simple and non-technical explanations (as far as possible) would be appreciated.

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Faheem Mitha
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There are two big prongs of the out of Africa theory or whichever name you wish to call it.

Prong the first: fossil evidence. There are lots of different kinds of protohuman fossils. Homo erectus/ergaster are found all across Europe, Asia, and into Indonesia from about 1.5 million years or so until about 70 thousand years ago, where they stop showing up. Up until 125 thousand years ago anatomically modern humans were only found in Africa. About 70 thousand years ago they start showing up in the Middle East, and then they spread out roughly following these paths. The fossil record shows not-humans or almost-humans (Homo erectus mostly) for millions of years, then humans show up at a certain point and then there's loads of human fossils. This point gets more recent as you get further from East Africa.

Prong the second: genetic diversity. Starting with this paper (not sure if this is behind a paywall but the details are not important anyway) human mitochondrial DNA was compared using restriction enzyme mapping. This kind of mapping is super crude (roughly analogous to grinding pottery into a fine dust and grouping the dusts by color) and the original study was pretty limited by the computers available at the time. Nonetheless, they showed that the most genetically diverse geographical group was Africa, and furthermore that any two people from outside Africa are likely more closely related to each other than any two people inside Africa. Genetic diversity goes down the further away from East Africa you get, matching what you'd expect if humans hadn't been living there long. There have only been (very roughly) 60 generations since humans colonized New Zealand. All Maori people, therefore, are at most 58th cousins. All indigenous Australians are at most 1998th cousins. All Africans are at most 8000th cousins.

More recently we've done proper sequencing on the mitochondrial DNA (actually knowing whats written there) and that's allowed much more precise estimation of this sort of thing, and estimation of the timeframes involved. The genetic evidence backs up the fossil record, by and large. (There are a few interesting examples where interbreeding with Neanderthals and maybe Denisovans can be detected, and that the settlement of the middle east by modern humans about 125 thousand years ago failed. They left no descendants.)

Fun side consideration (warning, extra science, proceed with caution): all humans inherit their mitochondria from their mother, and all males inherit their Y-chromosomes from their father. This lets us trace matrilineal lines with a relatively high degree of accuracy, since mitochondria reproduce asexually. To clarify: your matrilineal line is your mother, and her mother, and so on. Your paternal grandmother is not part of your matrilineal line, nor are your aunts or sisters. These lines are statistically guaranteed to eventually converge on a single woman whose mitochondrial descendants now live in all of us. The reasons for this are complicated. There are actually lots of these women, since she inherited her mitochondria from her mother, ad infinitum (well, not infinitum. Ad failed-endocytosis-of-an-alpha-proteobacterium-leading-to-mitochondria doesn't roll off the tongue though). Science has tight estimates on the most recent mitochondrial ancestor, or "Mitochondrial Eve". To be clear: there were loads of other women alive at the time, and we inherit a lot of their non-mitochondrial DNA. Over millennia all those other women had fewer daughters, and their matrilineal lines died out. She lived about 200 thousand years ago, plus minus 20 thousand years. As a mathematical guarantee we'll never find her body or have a really tight estimate on how long ago she lived, but she's genetically guaranteed to exist. The same principle applies to patrilineal inheritance, but the estimates for the most recent ancestor of all Y-chromosomes are a little looser (200-300 thousand years ago). See here for a more detailed handling of Eve and what's going on there.

In sum: old fossils only in Africa, everyone outside Africa is only 2800th cousins with each other or less, but intra-African relatedness is less than half that (more than twice as far away genetically).

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Resonating
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  • Does this mean mitochondrial eve had multiple kids with multiple men? – MastaBaba Jul 20 '15 at 22:55
  • No, mitochondrial eve isn't real women, it is computer reconstruction according to genetic theories, mutation rate, inheritance and so on. Change one parameter (estimated effective population size, although that one have loose connection with real population size as well) and you have different estimate of Mitochondrial Eve. – Colombo Jul 21 '15 at 00:37
  • Why would matriarchial lines terminate at one person in the past instead of there being multiple lines? That doesn't make a lot of sense to me. It would either mean that matriarchial lines never crossed, or that initially there was only a single woman. Does the term "mitochondrial eve" actually refer to a group of women? – yters Jul 21 '15 at 02:19
  • It's not that all ancestral lines go through Mitochondrial Eve's exclusively, it's that she's the most recent person which everyone's 'set of all ancestors going back X years' can be expected to have her in it. – Catalept Jul 21 '15 at 04:19
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    @yters It's not made explicit in the answer, but mitochondrial DNA is passed almost exclusively through the female line (sperm usually only has one or two mitochondria, if any at all, while the ovum has more like fourty). There has been many lines of mitochondrial DNA, but as soon as a mother only had sons, her line was broken. Add to this how many near-extinction catastrophes humans went through (apparently, there's been times when there were only a few thousand humans on the planet!), and it isn't all that suspicious that you can trace one line that's running in all of us. Others died out. – Luaan Jul 21 '15 at 06:52
  • @yters Mitochondrial eve is not a person or a group of persons (in fact I hate these creationistic connotations). It is basically a phylogenetic calculation of the Last Common Ancestor of human mitochondria (which are specifically maternally inherited). IMO addressing to this concept as her/she can be highly misleading. Resonating: you should clarify this in your answer. – WYSIWYG Jul 21 '15 at 10:29
  • @Catalept that's not quite right, you're thinking of something else I think. Most recent common female ancestor, maybe? – Resonating Jul 21 '15 at 13:21
  • @Colombo Mitochondrial Eve is a single human female who is guaranteed to have existed based on what we know about how genetics works. You're confusing imprecision in the measurement technique for imprecision in the measured thing. If you had a time machine Mitochondrial Eve would be time-consuming but trivial to locate precisely. – Resonating Jul 21 '15 at 13:35
  • Mitochondrial Eve was called Hera. – Lightness Races in Orbit Jul 21 '15 at 15:59
  • @LightnessRacesinOrbit We're wondering (in chat) what you meant by that. Enlighten us, please. – Faheem Mitha Jul 21 '15 at 16:49
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    @Faheem: https://en.wikipedia.org/wiki/Hera_Agathon – Lightness Races in Orbit Jul 21 '15 at 17:01
  • @LightnessRacesinOrbit I see. Thanks for the clarification. I just remarked to Resonating that apparently we don't watch enough Battlestar. – Faheem Mitha Jul 21 '15 at 17:09
  • @FaheemMitha: :) – Lightness Races in Orbit Jul 21 '15 at 17:12
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    @Luaan, thanks that makes more sense. So, to restate what you are saying, there is a single female in the past that everyone shares mitochondrial DNA with. It still doesn't make sense that there has to be a single female. Mathematically, it seems there could be many "mitochondrial eves" from the same period. – yters Jul 21 '15 at 18:24
  • @WYSIWYG, correct me if wrong, but you are still saying this calculation is meant to pick out a single female individual in the past with whom everyone shares their mitochondrial DNA, correct? The calculation is not just a mathematical construct, but is meant to refer to a real individual in the past? – yters Jul 21 '15 at 18:26
  • @yters There could have been a person but mitochondrial genome can also accumulate mutations and the rate of mitochondrial mutations is higher than that of the nuclear genome. All I said is that 'mitochondrial eve' is a hypothetical person. I said that I think the term "eve" can be misleading. – WYSIWYG Jul 21 '15 at 18:41
  • @yters Yes and no. At any given point, there's a set of women that are the 'mitochondrial eves' of that point in time. These women between them are mitochondrial ancestors of all living humans. At this point in time, it's all mothers. If you go back further it's all maternal grandmothers. If you go back far enough the set keeps getting smaller, until it's one person. Any further back and it stays one person. Come to chat, we can back and forth faster there. – Resonating Jul 21 '15 at 22:28
  • @Resonating If I had a time machine, tracing ancestral lines would indeed be an interesting use for it... – Michael Jul 22 '15 at 01:29
  • @yters Note how the single mitochondrial "ancestor" appears so long after humans did. In the beginning, there have been a lot of different mitochondrial lines - if we take the Homo genus as a whole, that's been around 10 million years, if we go from the point where modern humans diverged from Neanderthals that's around 500 thousand years. Interestingly, anatomically modern humans actually arose around the same time as MitE. But even at the time of MitE, there has been tons of different mitochondrial lines. It's just that one won out in the end - all modern Mit lines are descendants of MitE. – Luaan Jul 22 '15 at 06:58
  • @yters It is indeed possible that there wouldn't be a MitE for Homo sapiens sapiens, for example. But eventually, outside of a given species, you do come to a single ancestor - maybe it was some hominid, or a fish. But with each generation, the probability of only having a single (mitochondrial) ancestor grows. If you only have sons, your MitE line just dies out, ditto if you die. As far as I know, we don't actually know if MitE was an anatomically modern human - it's not like we have her fossillised remains. Her identity is based on probabilistic analysis of human mitochondrial lines. – Luaan Jul 22 '15 at 07:03
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    @yters In any case, this is really getting dragged out. Perhaps you'd like to post a question specifically about Mitochondrial Eve? I'm sure there's plenty of people who can explain this better than me - I'm no expert on human evolution or biology in general. – Luaan Jul 22 '15 at 07:05
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People previously argued for a multiregional theory in which humans arose from many different places on the globe, but have come to the conclusion that humans came originally from Africa. I believe that their reasoning is that the oldest skulls found have been found solely in Africa. This article expands on my answer if you are interested:

http://news.nationalgeographic.com/news/2007/07/070718-african-origin.html

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