Excellent question! There are some who propose that the first life came from a meteorite which struck earth.
But there is another theory that involves evolution. the first life started when some organic molecules got trapped together within a membrane. When some RNA got trapped inside the membrane too the organism could use it to become more complex. These very first organisms were called protobionts, it is these organisms that evolved to become prokaryotes (microbes, bacteria etc) and then eukaryotes (which have an enclosed DNA in a nucleus) which eventually evolved into multicell organisms.
I had a lecture about this years ago. It is believed that RNA was basically able to form structures that acted like enzymes, have a look at this http://en.wikipedia.org/wiki/RNA_world it expalins it alot better than I can.
But where did the RNA come from? Well chemists have shown that if you put all the ingredients needed to make RNA on a bullet and fire it at metal at the sme speed an asteroid goes when it hits earth, then you can form RNA, or something very similar. These ingredients have also been found on in space, as well as on earth, so the asteroid theory could be right 🙂
Microbes are a diverse class of organisms that can’t be seen without a microscope. These include fungi, viruses and protozoa, but perhaps the most well-known group are bacteria.
Bacertia are among the simplest forms of life. Single-celled organisms, enclosed by a cell membrane, they don’t have any of the internal structures that you might see in an animal cell – no nucleus, no mitochondria – the small amount of DNA they do carry is floating free in the cytoplasm.
Bacteria were one of the earliest forms of life to evolve – some put the date at about 4 billion years ago, in a warm little pond somewhere. These early bacteria were probably just little bags of chemicals – DNA/RNA and maybe some proteins, surrounded by a simple cell membrane. It was probably just a matter of chance – Ollie’s mentioned the RNA world. RNA is capable of storing information like DNA, but also of catalysing reactions like enzymes. If a few of these chemicals became enclosed by an early form of cell membrane, they’d be more protected, and the RNA would be more likely to replicate itself. A very early form of evolution.
From these humble beginnings, bacteria flourished. They can evolve much more quickly than multi-cellular species, so are good at adapting to lots of different environments They’re found everywhere on earth: from the ground upwards. They’re in the soil, in the ocean, in the freezing snow of the arctic, and around volcanoes where it’s too hot for anything else to live. There’s even a load in your intestines that help you digest your food!