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Evolution

I was just watching a program about evolution and was thinking about the many arguments that question the amount of time it would take single-cell organisms to evolve into us. My question is, considering that our brains can signal our bodies to react to infections, conditions, etc., is there any chance that our brains could likewise send a signal programming which genetic traits we'd like to pass on to the next generation?

-A curious adult from California

December 15, 2010

Not long ago, the answer to your question was a clear "no." Genetic traits get passed on randomly to the next generation. There was no evidence that parents could choose which genetic traits get passed on.

Lately, though, studies have shown something surprising: parents can affect how their kids use some of their genes! A well-known example is a study done on children born after the Dutch famine in World War II.

Hungry moms had kids that turned on a gene that prepared them for hunger. The gene made the babies use the energy in food more efficiently by growing more slowly.

Two recent studies on mice show something similar. In both studies, scientists overfed mice making them obese and diabetic. These mice then went on to have pups.

These pups then ate a normal, healthy diet. Surprisingly, the pups still became diabetic more often than if their parents weren't obese. Again, the parent's food had affected the way their offspring used their genes.

But none of this is brain related. And they aren't really evolution either. Mom's conditions affected the next generation's genes but it did not change them. Evolution requires that a gene be changed in a way that gets passed on reliably, generation after generation.

These changes are not that -- they are really just chemical add-ons. These chemicals can come on or off, depending on various conditions. This system of chemical DNA changes is called epigenetics.

True evolution requires mutations, which are more or less permanent changes in the DNA. It also needs some change in a species' environment and lots of time.

Evolution Happens in Small Steps Over a Long Time


Click the image to see a larger,
more detailed geological clock.

It took a very, very long time for a single cell to evolve into us. The image to the right shows this pretty clearly.

This geological clock is a representation of the roughly 4,550 million years that Earth has been around. Already by 4,000 million years ago, there is evidence of simple life. But it took 2,500 million more years just to evolve into something with more than one cell!

The first land animals appeared just 400 million years ago. Humanoids (like Neanderthals) have only been around a few million years. And our species, Homo sapiens, has only walked the planet a few hundred thousand years; we're evolutionary babies!

So it only took 500 million years for life to arise. But it took more than 4,000 million years for life to evolve from single cells into complex creatures.

During that long time, many of the genes we now use evolved. Once these genes came into being, evolution could pick up speed. It had more material to work with.

For example, once the genes for making a finger existed, evolution became easier. Getting more fingers was just a matter of switching on the gene a bit longer.

But how can you get a finger gene from the DNA of a single celled creature? From a build-up of DNA changes called mutations.

All life has DNA and passes it on to their offspring. But every new generation will always have a few mutations and other changes in their DNA. Most of the time, these mutations have no effect or are bad for you.

But every now and then, these changes will improve a gene or sometimes even create a new gene. An organism with better genetic traits will usually be more successful than others. The new, improved gene will start to spread through a population. Enough of these changes and eventually new species can arise.

Mutations are Random

An important point to make that gets at your question is that DNA changes are random. Specific mutations do not happen in response to something we do. Mutations happen by chance.

A famous experiment showed that bacteria have mutations that make them resistant to antibiotics all the time. But once you put them on antibiotics, all bacteria without the mutation die. This is exactly what happens during evolution.

Random mutations built up in bacterial genes. Some of these mutations made the bacteria resistant to antibiotics. Once antibiotics were introduced, only the resistant bacteria lived. Now a new trait is common in the population. Add a few more traits like this and eventually you'll have a new species.

This kind of natural selection (as it is called) is easy to imagine. What is harder to wrap your head around is how complex things can also evolve. But it can happen too. It just takes lots of little changes over a long period of time.

A very nice analogy to explain how this might happen with an easier to understand example is given by Dr. McDonald from the University of Delaware. On his website, he shows how a complex system (a mousetrap), might slowly evolve in small steps from something much simpler.

It is important to remember, that evolution is an explanation of how one life form might change into another. It does not explain how life itself was created. Evolution happens because of random changes to our DNA. These changes become common because they are beneficial or sometimes just because of blind luck. Enough of these changes will, over time, transform one species into another.

Undoubtedly, new discoveries will be made about how evolution works. And undoubtedly, some of our current ideas are wrong. So, just like the epigenetic trick by hungry mothers, there are ways in which evolution might find shortcuts; we just haven't discovered them yet!

Dr. Hinco Gierman, Stanford University

How evolution can work to form a clock.


What our parents do can affect how our genes get used. These mice have the same DNA.


How a complex system such as a mousetrap might evolve through simple steps.