Ask Erik: Episode Forty-Six

Here at Ask Erik we've spent a lot of time reading novels and comic books, playing video games, and watching television and movies in order to amass a deep vault of pop culture knowledge.  While constantly trying to still gather new information, it seems only prudent to share some of what we learned to help solve some of the world's greatest questions.

Will the next Olympics be any riskier or somehow safer than ones in large cities?  Do actors who are typecast for "bad guy" roles ever feel insulted by it?  Why can't people I know figure out that just because someone links to something on Facebook it doesn't mean it happened recently?  That girl accused of witchcraft in Oklahoma was in 2000, people, read the article!

...where was I?  Oh, right.  Once a week Erik tackles a question asked to him and tries to answer it in a method that handles the topic with the respect and attention it deserves.  Failing that, he'll at least try to make it funny so you don't regret the time spent reading it.

To Erik: From an evolutionary standpoint, why are we not all plant people, requiring only sun, water, and earth to live?

Well, first off, who's to say we're not?

Meet your oldest known distant relative, the stromatolite.  

These collections of bacteria thrive off of sunlight and figured out how to collect clay and build large columns (in their non-existent eyes) so that they could get more sunlight and conduct photosynthesis.  It's one of the oldest known living species on the planet, going back three billion years.  This organism is part of the reason we even have oxygen in the world to begin with, because back then there wasn't any in the atmosphere.

So how did we get from bacteria to my sitting at a computer typing about bacteria?

Well, it's a long, long, long procedure involving how oxygen got into the atmosphere but first filled the ocean, and then how the ocean actually rusted because of how much iron was in it, but we can skip ahead at least two thousand million years because during that time stromatolites were the dominant life form on the planet (take that, everything else that ever lived), and in fact, we can jump to about a billion years ago when the first sponges, seaweeds, and embryos started to show up in the fossil record (nature, at that point, finally had air to work with and each type of bacteria, as anybody who gets colds often knows, likes to work hard to constantly evolve and survive).  Then there's the massive first known ice age, and then we get to about 640 million years ago when the Ediacaran period began, and it's thanks to fossil records found in Australia that we know this was the first time period we can confirm multicellular life forms existed that were distinctively animal, not plant.

So, what happened during that time period?  Well, there's a few theories.

As far as I understand it, and I'm not an expert, there's two key differences between plant cells and animal cells.  Plant cells contain chloropasts, which are what allow them to convert sunlight into energy via photosynthesis.  Theories believe that the reason they have chloropasts is because of a symbiotic relationship between early (and I do mean early) plants and a cyanobacteria that provided the two a means of coexisting and evolving together.

Now, the neat thing about plants is that they're pretty much set for life with their basic needs met.  Sunlight, water, and dirt full of nutrients are all they need to flourish with few exceptions.  Animals, however, are both much more and much less complex.

See, while there are now arguments that plants actually do have nervous systems, the problem is that they don't have a central brain, like animals do.  Now, I know what you're saying:  "But Erik, it's a known fact that jellyfish don't have brains, and they're animals!"

And, well, you're right.

Look, if this was easy, I would've banged it out in one paragraph.

However, to bring this back around, there are enough cellular differences between plants and animals that stemmed from this early evolution millions and millions of years ago and there are things we do know about plants and animals that allow us to make a few conclusions:

1) Plants aren't built for extended uses of energy.  They don't have the same type of organs animals do, and while they are known for brief moments of movement (Venus fly traps and plants that retract when bothered), that's a very brief, twitchy reaction, a rather huge difference between that and running one city block.

2) Plant cell structures are built to restrict movement.  While you can bend a fern or a blade of grass, the larger a plant gets the harder it is to manipulate.  A sapling can bend a little, but nowhere near as intricately as your fingers or toes can wiggle.  Some plants manage to be huge and able to be bent (large vines in the jungle, for instance), but they're mostly known for hanging limply instead of wiggling themselves around like giant snakes.  Animal cells don't have that restricting cell wall, allowing for easier movement.

3) Just because we didn't evolve from an orange tree or a redwood doesn't mean we don't both have that same distant ancestor.  If anything, think of our evolution like this:  once all of the stromatolite bacteria finished flooding the ocean with oxygen (and using up all that iron left over from meteor strikes and the planet's volcanic activity), there was only one place for that oxygen to go: up.  And had the process continued without any other life coming along that could subsist on oxygen, there'd be some serious problems happening.  

If we wiped out all animal life and just left plants and bacteria that subsist on photosynthesis, you'd flood the atmosphere with oxygen with nothing to use it up except the plants themselves (fun fact: plant roots use oxygen and expel carbon dioxide because they can't use photosynthesis).  However, if we did leave bacteria around the world, this increase in oxygen would cause serious problems because bacteria thrive in oxygen-rich environments and would start wiping out plant species because the one limiting factor (oxygen availability) on their ability to breed is gone.

But let's go further, let's wipe out bacteria as well and simply stick with plant life.  Eventually, all of the carbon dioxide would be wiped out (save for a small amount), and the plants themselves would die off until the balance was once again restored...but without that bacteria around to decompose the plants, it would take much, much longer to happen.

So let's bring the bacteria back.  So we might have this steady balance of bacteria to eat dead plants and release CO2 and plants releasing oxygen to feed the bacteria until such a time as some new cosmic balance is laid out on the planet, but it begs the question "what were those other bacteria doing during that billion years that plants were evolving?"

Oh, right, they were evolving into a form that was mobile and could actually search for nutrients instead of being completely dependent upon nutrients finding their way to them.  That lovely oval up above is called Dickinsonia, while this guy:

Well, he's known as Spriggina, one of the first creatures believed to actually have a crude form of brain.

Thus, we have animals, and once the evolutionary growth of animals started, nothing short of another massive meteor strike was going to stop it.

So, it's certainly more complex than all this, and it involves looking and eukaryotic and prokaryotic cells, natural selection and genetic drift, and many, many other things that involve an advanced science degree to lay out, but I guess I can break it down thusly:

Animals and plants did have a distant, distant common ancestor.  The reason why we didn't come from plants is simply that early life did everything it could to survive, and all those attempts wound up creating all life as we know it (even if we don't understand it all yet), all specialized in its own way to survive amid everything else.