Tag: life

Evolution – Part Two

Credit United States Geological Survey – Public Domain – Tap for large original

Some years ago I published a series of articles about evolution in my local newspaper. It generated some interest and a spate of letters to the editor, and my publisher liked it. There was even a creationist who challenged me to a debate over it. I decided to reproduce it here. This is part two, which I called Tough Life. See also Part One and Part Three.

Life started on Earth just about as soon as it could. When the Solar System condensed out of a vast cloud of dust and gas about four and a half billion years ago, Earth was a molten globule. It was kept molten for a few hundred million years by a continuing bombardment of comets and planetoids among the thick debris. Eventually, as fewer collisions occurred, Earth cooled and its crust formed and hardened. It was still so hot that the huge quantity of water delivered by the comets was kept vaporized. Only after about 500 million years had it cooled enough for standing water to form on the surface. Not long after that we begin to see fossils of tiny organisms.

There is a question whether life formed through the evolution of chemistry on this planet, or arose from space-borne particles. Either way it’s been here for almost four billion years. The fact that it took root as soon as it could and has survived for so long both show how tough and persistent life is. It has had to be. There have been at least five major extinctions that we know of in the interim. There were probably more, but we haven’t unearthed the evidence yet. Some of the extinction events extinguished over ninety percent of the species on Earth. Once in a long while everything changes forever.

The first creatures were tiny, simple, single-celled organisms. They didn’t even have a nucleus, their DNA floating freely within the cell. From them evolved more complex forms of life, with DNA in a nucleus and other structures performing ever more complicated tasks. About two and a half billion years ago evolution produced a radical change. A microbe appeared which could use sunlight to synthesize food from water and carbon dioxide. Unfortunately for the existing life forms, the oxygen produced as a byproduct was a deadly poison. Things were changed forever.

About five hundred forty million years ago the abundance of free oxygen provided the energy for another radical change. Multicellular life arrived and proliferated in an explosion of diversity. It was a matter of time before some of it found a way to live on land, away from the competition and sharp teeth in the sea. It wasn’t long, in geological terms, before the land was covered in a riot of life. Evolution filled every niche with a profusion of species, including the majestic dinosaurs. Then the most famous mass extinction happened sixty-five million years ago. Dinosaurs were out and mammals were in.

Life took hold here early and has persisted by evolving and adapting to a state of permanent change.

rjb

How Did Life on Earth Begin

Today life has conquered every square inch of Earth, but when the planet formed it was a dead rock. How did life get started?

We have probably been wondering for millennia how life started on Earth. At some point we decided that it must have been done by one or more gods, and we’ve elaborated that hypothesis as it evolved over the ages. We’ve never tested it, though. No one has ever done an experiment to see if a god could create life. We just sort of said that they must have because, well, how else could it have happened? The judicious application of exclusion, torture and execution kept the questions to a minimum.

Eventually we reached a point where inquiry could no longer be peremptorily quashed, at least not everywhere all the time, and we began to look at the question in a more naturalistic way. That is, could life have arisen on Earth through the natural workings of physical elements and forces? The linked BBC article is a long, comprehensive look at the evolution of this inquiry over the last two hundred years or so. I strongly recommend reading it, but in the case of TL;DR, here’s the condensed version: We haven’t settled on a single theory yet, but we’ve certainly shown that’s it’s plausible.

From the BBC article:

The oldest known fossils are around 3.5 billion years old, 14 times the age of the oldest dinosaurs. But the fossil record may stretch back still further. For instance, in August 2016 researchers found what appear to be fossilised microbes dating back 3.7 billion years.

This means we can define the problem of the origin of life more precisely. Using only the materials and conditions found on the Earth over 3.5 billion years ago, we have to make a cell.

Before the 1800s, most people believed in “vitalism”. This is the intuitive idea that living things were endowed with a special, magical property that made them different from inanimate objects.

… the big biological breakthrough of the 19th Century was the theory of evolution, as developed by Charles Darwin and others.

The idea that life formed in a primordial soup of organic chemicals became known as the Oparin-Haldane hypothesis. It was neat and compelling, but there was one problem. There was no experimental evidence to back it up. This would not arrive for almost a quarter of a century.

The Miller-Urey experiment.

Miller connected a series of glass flasks and circulated four chemicals that he suspected were present on the early Earth: boiling water, hydrogen gas, ammonia and methane. He subjected the gases to repeated electric shocks, to simulate the lightning strikes that would have been a common occurrence on Earth so long ago.

And five more chapters follow. It’s really worth the read. But I think I can safely guarantee that the creationists won’t be convinced. They prefer the age of vitalism.

Source: The secret of how life on Earth began

For your interest, see my previous posts on Panspermia and life in the Solar System.

rjb

Alien Life on Titan

Photo credit - Cornell University Photography

Photo credit – Cornell University Photography

I thought I had published this article on solvent as the medium for life. I guess I was wrong. Today I was reminded of it by this article about some scientists who are trying to figure out what kind of life could evolve in liquid methane on Saturn’s moon Titan.

From the Eurekalert article:

“. . . many astronomers seek extraterrestrial life in what’s called the circumstellar habitable zone, the narrow band around the sun in which liquid water can exist. But what if cells weren’t based on water, but on methane, which has a much lower freezing point?”

“The azotosome (the material proposed for cellular life in liquid methane) is made from nitrogen, carbon and hydrogen molecules known to exist in the cryogenic seas of Titan, but shows the same stability and flexibility that Earth’s analogous liposome (the basis of our water based cells) does.”

The azotosome - Credit - James Stevenson

The azotosome – Credit – James Stevenson

And here’s my five year old article called “Solvent.”

On Earth the ultimate solvent is water. It lies on the planet in great swaths, covering almost three-quarters of it. But it doesn’t just lie there. It circulates around the continents in huge streams and mixes itself in small eddies. And it doesn’t just move horizontally. Water rises up into the atmosphere as a vapor and spreads out over the globe. In the air, water can take many forms. It can remain a vapor or it can condense out in tiny droplets or ice crystals, which can float in the atmosphere almost indefinitely. All the water will eventually complete the cycle many times over the eons, though. It will condense into drops or flakes or pellets and fall, some of it landing on the ground and finding its way back to the sea. While it’s doing all this it’s also acting as a solvent. Everywhere it goes it’s carrying all sorts of things that have dissolved in it. Water holds lots of interesting molecules that can engage in some creative acts of chemistry. And where there’s chemistry there are the raw materials for life. On Earth liquid water is the solvent that makes it possible.

The nagging question of whether there’s life on Mars hinges on the existence of water, preferably liquid, to support it. Were there Martian seas in the past? Is there water underground now? We know the planet has water thanks to the obvious polar ice caps, but we’re not sure if there’s enough to maintain a biological system. There’s encouragement in the fact that life exists on ice here on Earth. If Mars has plenty of permanent ice underground it might have a subterranean biosphere.

Farther out there are moons of Jupiter and Saturn that probably have global oceans under thick crusts of ice. Does any interesting chemistry happen there?

Also orbiting Saturn is a cloud-shrouded moon called Titan. It’s a large moon, about fifty percent bigger than our own, and bigger than the planet Mercury. Probes sent from Earth have studied it and found that it has plenty of useful molecules in its atmosphere and on its surface. It also has liquid methane oceans, lakes, rivers and rain. What kind of chemistry would go on there?

We’ve tacitly assumed that extraterrestrial life would be found, if at all, in the presence of water. Now some biologists are wondering if water’s required. Maybe some other solvent like liquid methane would do. While water is the ultimate solvent here, elsewhere it could be something else.

rjb

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