Theoretical origin of life; most important stages (Introduction)

by David Turell , Friday, September 02, 2016, 00:36 (3004 days ago) @ David Turell

This article lists key stages in the development of life on Earth:-https://www.washingtonpost.com/news/achenblog/wp/2016/09/01/the-4-biggest-milestones-in-the-history-of-life-on-earth/?wpisrc=nl_popns&wpmm=1-"The first and most obvious milestone was the origin of life. The details of that remain murky. Since life began, Earth has been transmogrified through cataclysmic events. The evidence of life's origin was eroded, or vanished in the recycling of the surface through plate tectonics. The only thing left are grains of zircon and other minerals that may have signatures of molecules that hint of the presence of life.-***-"Once life got rolling, the Next Big Thing was photosynthesis, says J. William Schopf, a legendary paleobiologist at the University of California at Los Angeles. Photosynthesis, he told us, enabled organisms to create their own food from solar energy and at the same time transform the biosphere by emitting oxygen. For roughly half the Earth's history, oxygen couldn't linger in the atmosphere because it kept interacting with other gases and surface elements. But gradually the oxygen “sinks” filled up and oxygen, created by living things, became a major constituent of the air. That, in turn, enabled the emergence of life that could use oxygen metabolically, which supercharged the complexity of Earthlife.-"The next big moment, Schopf said, was the appearance of sexual reproduction roughly a billion years ago. Until then, life merely cloned itself. Sexual reproduction is a technique for mixing genetic material rapidly in novel arrangements. That accelerated evolution, Schopf said. Soon enough we had multicellular organisms, followed by the Cambrian Explosion, 542 million years ago, in which all manner of hard-shelled creatures with elaborate body types evolved quickly.-***-"Then he amended his short list of things that matter: “I guess I would put intelligence in there as the fourth big thing that happened.”-***-"With only one example of life - the stuff we see on Earth - we don't really have a good, universally accepted definition of life. NASA some years ago defined life as “a self-sustaining chemical system capable of Darwinian evolution.” Not bad, but technically a single rabbit hopping around your garden is not alive, because by itself it can't reproduce.-“'I don't think anyone has a definition of life that is universally agreed upon. The best we can do is describe some attributes of life - and then someone will poke holes in that,” says Pamela Conrad, an astrobiologist at NASA. “We know pretty well what non-life is, so we can rule out the other stuff. The leftovers is life.'”-Comment: I think he pinned down the very big steps. What is surprising is the use of oxygen. It is nasty stuff: think of the dangers of giant fires; note that we try to moderate the damage by eating anti-oxidants. We don't use oxygen directly. Energy in cells is from conversion of ADP to ATP and back again, because the phosphate bonds are energy rich.