Endosymbiotic theory

Imagine you swallow a fish and start breathing under water!
You feast on a bear and become a world champion in wrestling or you gulp down a cheetah and voila, now you're way faster than Usain Bolt!
Impossible, right?
Pretty true for the multicellular setting but guess what, your cells, each and every one of it has already had the nerve to do such bewildering stuff. Besides, had they not shown such prodigious conduct, our existence would have been a sure impossibility.

To explain how our cells did this magic, we'd have to know some high school basics first. 'Some" would actually be more, just familiarize yourself with PROKARYOTES and EUKARYOTES.
In the simplest of the languages, PROKARYOTES are the primitive cells devoid of organelles (membrane bound structures or simply, organs of a cell). Say, a human without organs.
EUKARYOTES are the more modern type of cells having organelles. Note that our cells, that is human cells along with plant cells are eukaryotes. So the question goes down to "Whence did the eukaryotic cells come?" The answer is given by Endosymbiotic theory.

( Don't get too edgy on reading this name 'Endosymbiotic theory'. Science can go queer at times. I still remember the day when I first read the word reciprocal. In no time both the number and my mood turned the other way round! Read on! It's brilliant.) 

The endosymbiotic theory tells us how prokaryotic cells absorbed other prokaryotic cells (endocytosis) and apparently didn't digest them but decided to stay together (symbiosis) and thus form a cell (or a supercell) which could perform both the functions of the previously independently existing cells. Thus they evolved and followed the "survival of the fittest ideology".
( I hope that wasn't a pretty long sentence to understand. That's the simplest way I could incorporate all the important points)

Now for the word called ELABORATING.....

Okay, so let's get back to the 'no AD no BC, far more hell back era' earth. Only 3 kinds of cells existed back then, say Chintu, Bittu and Tinku ( no Tom Dick and Harry. Indian names are far more cooler and ..... well ....funnier!)
# Chintu had the ability to wrap its cell membrane around things at it's proximity and eventually absorb them.
# Bittu could use light energy to produce sugars. ( Yes, you genius! Bittu could perform photosynthesis!)
# Tinku could break down sugars and release energy.
So Chintu, Bittu and Tinku were living independently and peacefully in their village. They loved each other and things were going pretty fine. But every story has a good old villan and here it's the environment. Soon, three of them realized that with time, their environment was getting stronger and if things would go on like they were, they'd probably die pretty soon.
 "We can't become what we need to be by remaining what we are"
So they quickly took the above piece of advice and decided to do something about the problem. No superhero could come and help them so the only option left was to become one. So one fine day, Chintu, in order to survive, absorbed Bittu but couldn't digest him. Now Bittu was no less smart. He started to divide while living inside Chintu. So now we had a supercell (saktiman) which could still absorb other cells and moreover could perform photosynthesis.
Similarly, a supercell (Krish) which could break down sugars as an additional function was formed.
Then there were supercells which could perform photosynthesis and break down sugars as well. So cool! (Rajnikant)
These Saktimans, Krish and Rajnikants were nothing but eukaryotic cells - every cell in your body and each cell in your brain which is making you read this blog (more precisely The Krish)

HOW COOL IS THAT?

So that's how your cells managed to swallow other cells, evolve in over some million years, made you, and now is making you read how it did such a great bloody job!
YOU....yes YOU miss/mister should be PROUD!
Getting back to where I started - you 'absorbing' a bear or supposedly a horse and managing to co-exist with it, or you gulping a whole cheetah and expecting a symbiotic relationship, or you swallowing a range of different species and ending up whispering *my precious*? - let's just keep that to our very own Potters and the Lords of mighty rings!

Now for the more biologically oriented readers.
 Energy producing cell was engulfed and thus formed the mitochondria while photosynthetic cells formed chloroplasts.
There are basically 3 evidences to prove this theory:
1. If we destroy chloroplasts and mitochondria of a cell, new ones won't be formed. So that means cells can't produce them.
2. Chloroplasts and mitochondria have their own DNA and ribosomes. Both the DNA and ribosomes are similar to bacterial DNA and ribosomes.
3. Chloroplasts and Mitochondria are double membrane structures where their outer membrane resembles the eukaryotic membranes and the inner ones represents the bacterial membranes, thus giving the evidence of endocytosis.

"It is not the strongest of the species that survive, nor the most intelligent, but the one most responsive to change" - Charles Darwin.