# hypothetical math/population question

Discussion in 'Physics & Math' started by matthew809, Oct 14, 2011.

1. ### matthew809Registered Senior Member

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Excluding all variations, and assuming birth/death rates are exactly the same throughout history as they are today, how long ago did Adam and Eve exist?

3. ### hardaleeRegistered Senior Member

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Insufficent data.

5. ### drumbeatRegistered Senior Member

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6,970,000,000 in the world.
World population growth rate went down in 2009 to a 1.1% rise.

So multiply 6.97 billion by 98.9 until to get down to 2.
Or multiply 2 by 1.011 until you get nearly 7billion.

2 people multiplied by 1.011 250 times gives approx 27.

Not sure of how useful the answer is though, as population rates exploded in the last hundred years and are still high, although not as high as the 60s.

7. ### matthew809Registered Senior Member

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I'm not sure what you mean. 27 BC?

8. ### originHeading towards oblivionValued Senior Member

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Current population 7 billion people
World population growth rate 1.2%

Using your criteria and these numbers, I calculated that Adam and Eve started having kids about 1823 years ago or about 200 years after the days of Jesus. You might notice a slight inconsistency with this result. This strange result is because population growth is not linear as you suppose.

edited to add: This is of course not a linear function it is exponential. The flaw in the results are because we are assuming a constant growth rate.

Last edited: Oct 18, 2011
9. ### AlphaNumericFully ionizedRegistered Senior Member

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Never, because the bible isn't a literal account of history but a mistake of half truths, allegory and plain flat out bullshit.

On a slightly less literal reading of your question the question is one of exponentials. Do you know how to model the population dynamics given birth and death rates and a specific value of the population at a given time? I ask, rather than giving the answer, because I get the feeling from your posts you're asking homework questions and it's better if you're guided through it rather than just given the answer.

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I get ~180BC assuming a continuous growth rate of 1.2% and 6.9billion people right now.

11. ### originHeading towards oblivionValued Senior Member

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Hmmm. So we are at Jesus +/- 200yrs.

I could easily have made an error it was a very quick calc...:shrug:

12. ### drumbeatRegistered Senior Member

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No, you misunderstand me.
I only went as far as 250 years and it gave 27 people. You need to go a lot further to get 6.9 billion.
Herbbread and Origin seem to have gone the full distance in the calculation.

13. ### ScaryMonsterI’m the whispered word.Valued Senior Member

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And where did Cain's wife come from? How do you fit her into the calculation.

14. ### originHeading towards oblivionValued Senior Member

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I would prefer to wait a while before I said how I got that number because I think AlphaNumeric is probably right about the homework aspect and I don't think it would be right to 'show my work' for matthew809 benefit.

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Ooops, I mean 180 AC! Much closer now.

16. ### originHeading towards oblivionValued Senior Member

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I'll assume the homework was turned in by now...
Just use the classic population formula:

$P_f = P_i e^r^t$

Rearrange

$\frac{ln(P_f/P_i)}{r} = t$

Where $P_f$ is the final population 6 billion
$Pi$ is the intial population (Adam and Steve)
r is the rate .012
t is time in years

The goofy answer is because the assumption of a constant growth rate is flawed. My earlier post said the growth was linear which was retarded, it is of course expontential.

17. ### prometheusviva voce!Registered Senior Member

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A bit of background: It is assumed that the rate of change of the population $\frac{dP}{dt}$ is proportional to the population $P(t)$. Hence $\frac{dP}{dt} = k P$ the solution to which is $P = A e^{kt}$. A here is the initial population and k is a proportianality constant.