Evolution
Over time, the most successful variations may result in species splitting into entirely new ones. LUCA (last universal common ancestor) is the origin of all life that we know of on Earth. Over billions of years, small variations resulted in the catalogue of approximately 1.5 million unique species that exist today!
Explaining the divergence of 1.5 million different species is a lot to take in, but we can demonstrate how evolution works by studying just one body part – the eye:
Patch of cells
The most basic form of an “eye” is a patch of light sensitive cells, known as photoreceptors.
Eye cup
A variation occurred that caused the patch of cells to form a cup, which got deeper and deeper each time an offspring inherited the variation. This allowed the organism to sense the direction of light.
Simple pinhole camera
Eventually, it was no longer favourable to deepen the cup. Instead, the eye cup folded in leaving only a pinhole gap. The cavity filled with fluid, focusing light and allowing shape detection.
Primitive lens
A variation in the fluid resulted in a crystalline lens forming. This protects the eye and gives it the ability to focus light at different distances, thus sharpening the image formed.
Complex camera
Additional successful variations resulted in the eye shared by most vertebrates (including humans) and cephalopods (squid and octopus).
All examples of these eyes still exist in different organisms today. A complex camera eye does not benefit a cave beetle, maintaining light sensitive cells to navigate the dark caves it lives in. Organisms with the most complex eyes are also the organisms with the most complex brains, able to do something with the detailed information detected by the eye.