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microbial life to homo sapiens?


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I cannot see how we can get anywhere close to knowing the answer to that question. The human genome contains about 30,000 genes. That is : lengths of DNA that code for specific proteins.

 

The simplest micro-organism may have had only hundreds of genes. However, that does not mean there were 30,000 mutations. It is probable that genes mutated many times over the eons that led from micro-organism to human. Thus, a lower limit would be something like 100,000 mutations, but may have been many more.

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Estimating generations would be difficult and have a very big margin or error. A generation for a micro-organism can be as little as 20 minutes (bacteria in nutrient agar at 37 Celsius) up to a thousand years (slow living bacteria deep under ground). However, if we assume 10 generations in a day as an average, we can come up with a guesstimate.

 

The first 3 billions years of life on Earth was microscopic. Thus, at 10 per day, about 11 trillion generations. The last 500 million years relates to higher organisms with much longer generation times. If we assume an average generation time of a month, we get 6 billion generations. In total about 11.5 trillion generations.

 

Not accurate, and the plus or minus factor is massive, but it may serve to give you an idea.

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One of many problems is that some mutations have large changes on the genome. For example, a large stretch of DNA might be copied, possibly adding copies of multiple genes. DNA and retroviruses can insert themselves into DNA, adding an entire copy of themselves to the DNA. Some mutations can add or remove small bits of DNA, and some will change the DNA but not its length. Some mutations could just so happen to reverse previous mutations. It would be very complicated to try to give a reasonable guess.

 

edit: Oh, and large chunks of DNA can also be deleted.

Edited by Mr Skeptic
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Well, all of the things from a microscopic colony of yeast are alive. The first thing that really came would be bacteria. Now, it's just a absolutely huge number. And the amount of species that have died in between that we do not know of would add to THAT.

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