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The Age of Things
Using radiometric dating techniques, scientists have been able to estimate ages of various things.
- The universe - 10-20 billion years old
- Our solar system (including earth) - 4.5 billion old
- Life on earth - 4 billion years old
- Proto-hominids (e.g., Australopithecus) - first appeared 4.4 million years ago
- earliest Homo sapiens - first appeared 400,000 years ago
- modern Homo sapiens sapiens - first appeared 200,000 years ago
But we finally did get to the party.
Who's Who in Evolutionary Theory: A History
Remember: The Origin of Life is not the same as The Evolution of Life.
We have a long history of trying to figure out both.
Plato and Aristotle
About 2500 years ago, these Greek philosophers were positing that the world had always been as they saw it, with everything having been put there by a divine creator.
Aristotle created scala naturae, a representation of the hierarchy of life. He believed it was unchanging.
Most of us are familiar with the story of a supreme being creating the universe from an empty void.
Ancient Norse creation mythology recounts the joining of primal ice and fire in the the great void--Ginnungagap--that created the oceans, the world, the gods, and how those gods created animals and humans.
There are as many creation stories as there are cultures.
Problem is...each of these stories is different from the others.
So they can't all be true.
Intellectual Stirrings in the Renaissance
recently as the 1700's, biology was still studied in a framework of natural
theology, the notion that science should be dedicated to studying nature in
order to figure out the Grand Plan of the Creator.
Origin of Life: The Birth and Death of Some Popular Ideas
Ancient Romans thought that living organisms could spring, fully formed, from non-living matter by a process of spontaneous generation.
This was inspired by common observations:
- maggots were "generated" from rotting meat
- lice were "generated" from sweat
- eels, turtles were "generated" from sea sediments
- An Ancient Recipe for Making Mice (ca. 1650)
1. Place sweaty underwear and husks of wheat in an open-mouthed jar
2. Wait 21 days.
3. Sweat from the underwear will penetrate the wheat husks and turn them into mice!
And I'm not really sure what to say about this guy.
It took the work of more than one scientist to lay the idea of spontaneous generation to rest.
In 1668 this Italian physician performed
experiments designed to test spontaneous generation. He placed rotting meat in covered and uncovered jars, and
noted that maggots formed only in the uncovered jar. (Enlightenment: "Hey! Maggots are baby flies! They come from fly eggs!")
Anton van Leewenhoek
In 1676 this Dutch scientist invented the microscope,
revealing a microscopic world teeming with life.
spontaneous generation was revived (though on a smaller scale).
Curator of the King's Garden in Paris, this intellectual was the first European to propose that the earth and solar system had arisen due to natural processes, and that life itself had emerged from the earth.
He found it striking that the earth and living things were all composed of the same types of particles (elements). He didn't propose a mechanism for this emergence. But he was one of the first to propose an idea other than divine creation.
John Needham - Oops. Back to the Drawing Board.
By 1745, it was well understood that boiling killed microorganisms. To test whether they would appear spontaneously after boiling, Needham boiled broth, then covered it. A few days later, microorganisms grew.
This seemed to show that some "life force" resided in the broth that would allow spontaneous generation. However:
- He didn't boil long enough to kill bacterial resting stages (endospores)
- He allowed the broth to cool, uncovered, to room temperature before covering it allowing plenty of time for colonization.
This Italian priest suspected that microorganisms had entered the flask after Needham had boiled it, but before he had sealed it, entering through the air. In 1768, Spallanzani
- filled two flasks with broth
- left one uncovered, one covered
- removed the air from the covered flask, creating a partial vacuum
- boiled broth in both flasks
- only the uncovered one produced microorganisms.
However, his experiments were discounted because...
1. Others with less meticulous techniques could not replicate his results
2. Critics said that all he'd shown was that air was necessary for spontaneous generation.
Louis Pasteur - Let's end this.
In 1862 he convinced the doubting scientific
community, with his elegant Swan-necked flask experiments, that microorganisms did not arise via spontaneous generation.
- He demonstrated that broth boiled in a swan-neck flask was protected
airborne microbes by the curve in the flask's neck, which trapped invading
microbes. But if he broke off the swan
neck and waited a couple of days, microbe growth occurred. There was nothing "unfit" about boiled broth or the air around it: both were perfectly suitable for bacterial growth.
- This demonstrated that spontaneous generation was not occurring.
- Pasteur's experiments put the controversy permanently to rest, and
omne vivum e vivo ("all life from life") became the accepted aphorism.
Taking Pasteur's Work Out of Context to Contest Evolution
Unfortunately, some who are not fully educated about modern understanding of the Origin of Life have claimed that since Pasteur "disproved" spontaneous generation, that life cannot come from non-living matter, and hence, evolution cannot occur.
This is, quite simply, WRONG.
Miller-Urey: The Modern Understanding
In the 1950s, Stanley Miller and Harold Urey duplicated the conditions of the primordial earth in this now-famous apparatus:
(Click on the picture of Stanley Miller for more detailed information)
The initial experiments, using water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2), yielded the amino acids glycine and alanine. Miller noted that over years of experiments, the apparatus produced eleven of the known amino acids.
Later experimental runs using a Miller-Urey apparatus showed that the apparatus's original chemical components could react to produce hydrogen cyanide (HCN), formaldehyde (CH2O), acetylene and other biological molecules.
Later it was shown that formaldehyde can react with water to produce various sugars, including ribose, the sugar component of RNA.
In subsequent years, other researchers observed additional reactions in similar apparati that produced biological molecules.
In 1961, Joan Oro demonstrated that the nitrogenous base adenine could be constructed from hydrogen cyanide and ammonia.
In short, these important biological molecules were synthesized in Miller-Urey apparatus and in similar apparati used by other researchers:
- amino acids
- simple sugars, including ribose
- nitrogenous bases (some of the "letters" of nucleic acid language)
One critical feature of the Miller-Urey apparatus was the absence of oxygen, which was scarce in earth's earliest atmosphere. (Oxygen is highly reactive, and would have destroyed such large molecules if it had been present.)
Still, no one knew how those molecules had come together to produce life.
Recently, some researchers are getting a bit closer to witnessing the tiny steps to that amazing outcome.
Abiogenesis: Recent Advances
Today, research has revealed even more clearly how, at some point, Life must have originated from non-living matter.
Which is not exactly what some people think.
Once life got here, how did it change into the vast variety of forms we see today?
Many scientists have contributed pieces of the puzzle.
After life arose from non-living matter, how did it change into the vast variety of forms we see today?
a testable idea of how organisms could change from one generation to the next was
Jean Baptiste Lamarck
- In the late 1700's he concluded that organisms could change from generation to generation.
- He said these changes were driven by
interieurs ("felt need", Fr).
- He meant that animals could change from one generation to the next if their environment required change for the next generation's survival.
- He thought that traits acquired by an individual in its lifetime could be passed on to that individual's children (offspring):
- Giraffes had long necks because they had to stretch to reach food high in the trees.
Each generation developed a slightly longer neck from stretching, and passed the longer necks on to their babies.
- The big, beefy muscles developed by the Village Blacksmith with his physical labors would be expected to be passed on to his offspring.
Lamarck: Use it or Lose it
Lamarck believed that evolution was driven by "use vs. disuse"
We now know that--generally speaking--traits acquired during an organism's lifetime are NOT passed to its offspring. Mendel's and Darwin's work helped refute Lamarck's theory.
- a used structure will become larger, stronger and more important.
- a unused structure will atrophy and become vestigial.
- He argued that traits acquired during an organism's lifetime could be passed on to its offspring, and this was a driving force of evolution.
- His work, published in 1809, was widely accepted.
Lamarck Rises from the Grave?
The relatively recent discovery and understanding of epigenetic inheritance suggests that while Lamarck may not have quite hit the mark, not all inheritance is strictly Mendelian. Environment can play a role in how DNA is packaged and expressed, and some of those DNA re-arrangements are heritable.
At about the same time Lamarck was publishing his ideas, French anatomist Georges Cuvier
was developing the science of paleontology, the study of fossils.
- He noted that deeper strata of sedimentary rock had
diversity of organisms more different from present day life than more
recent strata. His reason: catastrophism.
- Cuvier was a
staunch creationist, and insisted that the various signs of change in the
fossil record were merely confirmation of catastrophic events such as
floods and massive destruction wrought by the Creator.
- He proposed that natural
disasters had wiped the earth clear of life (locally), and that new life
had migrated into the devastated places from surrounding areas. (Most early European scientists were creationists
who believed--at least nominally--that the world was made in seven days by a supernatural supreme being.)
Among these scientist creationists was a young student named...
One of the most influential scientists of all time, Darwin elucidated his theory of how evolution proceeds in his (then) controversial work, On the Origin of Species by Means of Natural
Selection (1859). Origin of Species is arguably the most important
biological work ever written, and most modern biology is framed in the
context of evolution by natural selection.
Not all evolution is strictly Darwinian.
In 1968, Mootoo Kimura published his Neutral Theory of Molecular Evolution. In essence, his theory states that random, neutral changes in DNA at the molecular level can have profound evolutionary consequences over time. The rate at which these changes happen depends on population size. But Genetic Drift is now understood to play a major role in evolutionary change in living populations.
Research in this area is ongoing.
Here's a nice overview of some of what we know about the Facts of Evolution.