Saturday, February 25, 2006

The history of life

The universe is thought to be 13.7 billion years old. Matter and energy can not be created or destroyed - they can only change forms or convert from one to another.

By implication, that means you are 13.7 bilion years old. Or more precisely, the matter that comprises your body has existed, in some form or other, for 13.7 billion years old. This matter is constantly being recyled and replaced with new matter, matter that is also 13.7 billion years old. What your age represents is the length of time that the atomic pattern that is the blueprint of your being has been in the world.

At the start of the universe the only elements that existed were hydrogen and helium. All the other elements, including the organic elements that are the building blocks of life on Earth, were produced inside of stars. This means that at some point, the carbon and nitrogen and oxygen that is the bulk of your body (excluding hydrogen) was formed billions of years ago inside of a star by the process of nucleosynthesis. As Carl Sagan liked to muse, we are quite literally made of star dust.

The Earth formed about 4.5 billion years ago. One billion years later, evidence of life first emerged. There currently is no accepted theory as to how life began, but I suspect time will bare out the finding that life began with some sort of autocatalytic process where proteins that filled the oceans began to self-replicate. If this turns out to be correct, this means we are the Earth come alive.

Speculation aside, you represent an unbroken chain of life stretching back to the time when life first appeared. It took 3.5 billion years of evolution to arive at you. You represent genes that have traveled billions of years, changing and evolving along the way, to arrive in combination at the production of you.

All life on Earth shares as a common ancestor that first life form that rose in the sea. That means you are related to every living thing on the planet. It is a favorite habit of Creationists to hang their argument of personal incredulity upon the notion that we evolved from monkeys (we didn't, we share a common simian ancestor with them) but I've never understood this, because if you're going to make such an argument (which is a logical error, anyways) one can find much more outrageous claims from evolutionary theory. Yet these claims are true.

The common ancestor of all multi-celled animals was probably something resembling a sea sponge. Yes, one of your great........great grandparents may have been a sponge. Strange but true.

Approximately 440 million years ago one of your grandparents was swimming in the ocean as a fish. Thankfully for you, he managed to not get eaten before reproducing and passing on the genes that would eventually become you along their billion years journey.

One hundred millions later - 340 million years ago - those genes had evolved and brought your ancestor out of the sea. This ancestor was some sort of amphibian.

Another hundred million years marked a tough time for your ancestor, as at the end of the Permian there was a mass extinction in which 90 - 95 % of life on Earth was wiped out. This grandparent was now a mammal-like reptile, probably something close to a therapsid. If he hadn't managed to survive the greatest extinction in Earth's history, you would not be here today.

More time passes. Its the age of dinosaurs. Your ancestor is now a small, shrew-like mammal. And around 65 million years ago, he manages to survive a meteorite strike that was so cataclysmic that 85% of all life went extinct. According to Richard Dawkins, the sound of the collision would have been so thundering that most life on the planet would have been struck deaf. Your possibly deaf, possibly blind, possibly burned ancestor dodged a proverbial bullet, fortunately.

This great .... great grandparent of yours survived and its descendents proceeded to evolve, slowly and slightly, so slowly and slightly that our minds, which have a habit of trying to view the world in discrete categories, have a difficult time grasping how such changes could ever occur. But occur they did, and around 40 million years ago - 25 million years after the great escape of your other ancestor - your ancestor was now swinging from a tree. He was something similar to a monkey.

It took another 40 million years of evolution to get from him to you. That's 40 million more years of your direct predecessors managing to win the game of life and successfully pass on their genetic material to the next generation.

If you think this is remotely interesting, then check out The Ancestor's Tale by Richard Dawkins. Its written as a backwards pilgrimage through time, with humans being joined along the way at rendevous points in history by other living organisms where they share a common ancestor (concestor) with us.

4 comments:

Alan said...

First, just to let you know your writings are not in vain, lately I have found myself checking your blog regularly. I discovered it by following a link from one of your comments on Massimo's Rationally Speaking blog , but find yours actually a little more even keeled -- don't tell Massimo :). Unfortunately, I am one of the choir since I consider myself a freethinking, rational, secular humanist (though a bit more a fan of free markets and capitalism than Massimo perhaps) who values objective truth (where possible) and honesty in arguments above all.

One thought experiment I propose to Creationists who argue from the perspective of personal incredulity involves looking at photographs of themselves. Assume that you have a photograph from each day of your life and you laid them end to end. Assuming you were 80 years old that would be 29,200 photographs. I then ask the evolution doubter to note that there is no discernable difference between any two adjacent photographs -- even between 50 adjacent photographs or more. I ask them at what magical point did the infant become a child, a teenager, a man, an old man?

There is no evidence of change, it is so slight.

The change from species to species is even more slight, yet as with the photographs, there is over the long haul definite and undeniable change.

Of course its only an analogy, but I think it helps an open minded person grappling with evolution and the magnitude of time.

Dawkins wrote something similar involving mothers and daughters holding hands in a line. Two lines actually. I'll see if I can find it.

Alan

Alan said...

Ok, I found this on a message board. I'm not sure if this is from Ancestor's Tale or another Dawkin's work:

"Happenings are sometimes organised at which thousands of people hold hands and form a human chain, say from coast to coast of the United States, in aid of some cause or charity. Let us imagine setting one up along the equator, across the width of our home continent of Africa. It is a special kind of chain, involving parents and children, and we will have to play tricks with time in order to imagine it. You stand on the shore of the Indian Ocean in southern Somalia, facing north, and in your left hand you hold the right hand of your mother. In turn she holds the hand of her mother, your grandmother. Your grandmother holds her mother's hand, and so on. The chain wends its way up the beach, into the arid scrubland and westwards on towards the Kenya border.

"How far do we have to go until we reach our common ancestor with the chimpanzees? It is a surprisingly short way. Allowing one yard per person, we arrive at the ancestor we share with chimpanzees in under 300 miles. We have hardly started to cross the continent; we are still not half way to the Great Rift Valley. The ancestor is standing well to the east of Mount Kenya, and holding in her hand an entire chain of her lineal descendants, culminating in you standing on the Somali beach.

"The daughter that she is holding in her right hand is the one from whom we are descended. Now the arch-ancestress turns eastward to face the coast, and with her left hand grasps her other daughter, the one from whom the chimpanzees are descended (or son, of course, but let's stick to females for convenience). The two sisters are facing one another, and each holding their mother by the hand. Now the second daughter, the chimpanzee ancestress, holds her daughter's hand, and a new chain is formed, proceeding back towards the coast. First cousin faces first cousin, second cousin faces second cousin, and so on. By the time the folded-back chain has reached the coast again, it consists of modern chimpanzees. You are face to face with your chimpanzee cousin, and you are joined to her by an unbroken chain of mothers holding hands with daughters. If you walked up the line like an inspecting general--past Homo erectus, Homo habilis, perhaps Australopithecus afarensis--and down again the other side (the intermediates on the chimpanzee side are unnamed because, as it happens, no fossils have been found), you would nowhere find any sharp discontinuity. Daughters would resemble mothers just as much (or as little) as they always do. Mothers would love daughters, and feel affinity with them, just as they always do. And this hand-in-hand continuum, joining us seamlessly to chimpanzees, is so short that it barely makes it past the hinterland of Africa, the mother continent."

Hume's Ghost said...

Best meme in Ancestor's Tale is the tyranny of a discrete mind. (i need to do a post on Zeno's motion paradox.)

And I have to say, your praise is higly appreciated. Massimo's Free Inquiry column was one of my biggest inspiratations to start this blog.

Jen said...

Very nice post. That's also how I think of the universe. I just found your blog, I've bookmarked you to come back and ready more!