Note: Evolution does not create a single, clear path of change for a species. It creates many branches, like a tree. What this idea does is start with our species at the very end of one branch, and follows it back to the trunk.
We can no longer envision a simple unilinear model of our evolution. There were many expansions, contractions, and extinctions.
One generation is just a single short step in a much, much longer journey called 'evolution'.
Look at how similar one generation is to the next. Here's a mother and daughter at the same age.
Suppose your father... walked into this room at the ordinary human pace of walking. And suppose just behind him was his father. How long would we have to wait before the ancestor who enters the now-open door is a creature who normally walked on all fours? The answer is a week.
- Carl Sagan, The Varieties of Scientific Experience: A Personal View of the Search for God (2006)
Check out this page, maybe one of the best on Wikipedia. Scroll down a bit to get pictures.
This is the human species Graecopithecus who lived 7 million years ago, and may be our closest ancestor who walked on all fours (though the debate is ongoing).
But even Graecopithecus is a relatively recent.
65 million years ago, our ancestor might have looked something like this, a small, insect-eating animal with a long, furry tail:
300 million years ago Archaeothyris, a medium-sized lizard, was our ancestor.
380 million years ago, our ancestor was called Elpistostege
Image by Chase Stone
3.8 billion years ago, a single cell called Luca.
The chemical processes of the Earth.
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