Is there a geneticist in the house?

J. Marks said:

I have a tough question that I have not been able to find an answer to ANYWHERE!

Would someone from the time of Jesus Christ or Alexander the Great have noticable genetic differences from modern humans? If not, how far back in human history would a geneticist have to go before those differences started showing up?

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I'm not a geneticist, but I am a geologist specializing in paleontology, and the short answer is No. The human genome is extremely narrow in variance, and geneticists, based on DNA studies I don't pretend to understand, think that we modern humans have a narrow genetic origin due to some major constriction in the population about 75,000 years ago. From what I've read, the thinking is that the entire population of humans on the planet may have shrunk to as little as 3,000, and all 7 billion of us have derived from that minuscule group.

Of great interest is that the last colossal megavolcanic eruption known happened around 74,000 years ago, in Sumatra (Toba caldera). An event of that scale would have had an enormous impact on global climate for perhaps decades after, and likewise on the biology of the planet.

So you'd probably have to go back on the order of 100,000 years to see significant genetic variation in the human population. Although Neanderthals, who may or may not have constituted a separate species from Homo sapiens, existed up until about 40,000 years ago, as I understand.

But any human population within the scope of written historic time would probably be indistinguishable from the modern population in terms of genetics.

caw

Other than transcription errors etc the entire populations genotype changes only by random mutation and these mutation are often non-unique (have happened in the past). And the mutations that do occur are only transmitted to offspring of that person. For that reason, a few thousand years is essentially no time at all.

I'd agree that there would be no way to tell. Major modifications to the genome happen to humans very little- you'd have to go back to when species are splitting. There are other cool ways to tell things from the genes though.

Can you give a better idea what you want to use markers for? Maybe we can give you suggestions that would help change but not make you make a major rewrite

(PS I am a scientist with a PhD in molecular biology)

You'd have to go back to as far as the first split from the 'early man' which led to Homo sapiens and even then the differences may not be that great.

I heard that it was 10,000 humans who migrated out of Africa who were the basis for every single human on the planet now. All variations in phenotype are more due to adaptations to environment (causing some genes to express more strongly than others) rather than genonic differences (i.e. we have the same DNA sequence as our ancestors but some of us express some of the genes more than others).

A lot of current research is actually looking at post translational changes to expressed proteins - the effects of things like heat shock proteins - to explain why differences can occur so quickly (i.e. why people who are chinese have that particular shape of eye or why those of African origin have that skin colour).

However, there are issues with researching this stuff (a text book I saw recently had a question for students asking them to debate the morals of research into why racial differences occur - could it count as discrimination?)

The deal with the anyone circa the Roman Empire is that they moved people around - a lot - and in large numbers. Before that, you might have been able to pinpoint regions based on genetic data because the incidence of genetic quirks would be localized, but after massive long term troop movements, and the resulting mix of local people with transplanted Roman soldiers, those hot spots for certain recessive expression would begin to spread out.

Even with that, you still wouldn't be able to take the remains of a Roman soldier, test his DNA, and say "Hey! This guy lived during the Roman Empire!" without some contextual clues like clothing, weapons, etc.

since it is sci-fi, maybe you could strech a bit a science. Have you ever heard of the book, the seven daughters of eve? It basically traces mankind back to seven women since they use mitochondrial DNA (which is passed from mother to child) Maybe you could just say that Alexander came from a rarer sample. I doubt in reality he is, but it would be an easier stretch than what you have proposed.

McMich beat me to it. I was going to suggest mitochondrial DNA and some hand waving.

You could say that his Y chromosome haplogroup is a subclade of I2 that is only known from archaeological samples, and is thought to be extinct in the modern population.

First, I am not an expert at all. And I'm not sure if it will be applicable in this specific instance, but doesn't the mitochondrial DNA allows scientist to use it as a molecular clock so that they can track mutations (on mitochondrial DNA) in populations back in time?

Alexander's mitochondrial DNA should therefor have less mutations compared with modern mitochondrial DNA. It is however possible that 2000 years is too narrow a time window for the technique. I've only seen it be used of far greater time-scales in documentaries.

But I reckon it would be a stretch why any scientist would look at the mitochondrial DNA so closely when he has no reason to specifically look for anomalies there in the first place.

Not to be a wet blanket, but you can't clone someone from mitochondrial DNA. You need the nuclear DNA which degrades uber fast. The paleolithic retrievals are in regard to mitochondrial DNA, except in still frozen carcasses like the high mountains and Arctic and even then, getting nuclear DNA is a royal pain.

Also it's only a little over 100 generations to get back to Alexander of Macedon. Unless there had been bottlenecking or population isolation in the meantime, it's doubtful that you could use DNA to say anything more than, "His family most came from this region."

Lillie's idea is most likely. There were, and sill are, genetic isolates around the world, and those sometimes disappear due to genetic drift.

The point of mitochondrial DNA is that it is the DNA of the orgasm which (according to some theories) infected primitive cells and entered into a symbiotic relationship which eventually led to it becoming an organelle. The whole concept of 'Mitochondrial eve' came from the fact that they tested a lot of women and found that they all had the same mitochondrial DNA which suggested that they all descended from the same woman (at least according to the tabloid media hyperbole). In actual fact, the more likely case is that they all descended from a pool of 5000 women who all shared the same mitochondrial DNA (or at least the majoity did, the chances are there could be some women out there with different mitochondrial DNA but they have not been found yet).

This DNA does not change with generations as much as nuclear DNA because it does not undergo Meiosis which is the biggest source of variation in the genome. You can therefore use it to trace generations back a long way.

If you want to look at the spread of an ancient DNA to the modern world, there was a study here in the UK which tested people all over the country to see how many shared a similar DNA profile to people in Norway in order to assess how far the Viking invasions and the later Danelaw incursions spread. It was a surprising result with matches popping up all over the place (but then maybe not so surprising given the amount we have migrated in the modern day...). Reading up on this study may give you an insight into how modern genetic techniques can and have been used as well as examining the spread of one particular set of genetic markers.

areteus said:

DNA of the orgasm

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This sounds like it would make either a good documentary about sex or a corny self-help book.

Snick said:

Lillie's iea is most likely. There were, an sill are, genetic isolates around the world, and those sometimes disappear due to genetic drift.

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Lol.
I just got curious and looked stuff up on Wikipedia. Didn't I learn some nice big words?
Subclade. It's like a sandwich, but with people in it!

Lillie said:

Subclade. It's like a sandwich, but with people in it!

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Are you writing a cookbook for cannibals?

If your book is a peer reviewed non-fiction book and your audience is geneticists then I'd worry.... If it's fiction for a general audience (and you can't get an answer on the WWW) then make your own plausable answer. The beauty of fiction writing is that we can take some fact and throw in plausible fiction then blur the lines between them.
Go with no genetic difference.... or go with a subtle genetic difference if it plays into your storyline. Have fun with it!