Life is a funny old thing when you come to think about it, not the least that it can be hard to define just what it is that sets animate things apart from their inanimate counterparts. Take snowflakes and crystals, for example. When we watch either of these “growing” using stop motion photography, it’s hard to shake the idea that they resemble living organisms, even though we know intellectually that they aren’t. What’s the largest living creature of which we are cognizant? Well, in the animal kingdom we have the blue whale, the biggest example of which we’ve seen thus far weighed in at 190 tons (173 tonnes). But let’s not discount Armillaria ostoyae, which is a species of plant-pathogenic fungus. The largest specimen discovered thus far, which is colloquially known as the “Humongous fungus,” covers 3.7 square miles in the Oregon Malheur National Forest. This little scamp is estimated to weigh in at around 35,000 tons (~32,000 tonnes).
Life is such a precious thing (Click image to see a larger version — Image source: pixabay.com)
Of course, we also have the Gaia hypothesis, which is the idea that all living matter on the earth collectively defines and regulates the material conditions necessary for the continuance of life, thereby likening the biosphere to a vast self-regulating organism. According to this article, “Every living thing on Earth — from the tiniest bacteria to a mighty redwood tree — weighs a combined 550 gigatons when removing water from the equation.” What’s the smallest living creature? Well, a parasitic bacterium called Mycoplasma genitalium is currently believed to be the smallest known organism capable of independent growth and reproduction, but who knows what we’ll discover tomorrow? And then there’s the perennial question as to whether viruses are alive. Unlike bacteria, plants, and animals that contain cellular mechanisms that allow them to self-replicate, viruses can’t replicate on their own; instead, they have to invade and subvert the cells in a living organism in order to replicate. Of course, a mule (the offspring of a male donkey and female horse) is sterile and cannot replicate, so does this mean it also is not alive? Is life defined by some sort of spiritual component? The ancient Greek philosopher Aristotle (384–322 BC) was of the opinion that all living things have one of three types of soul: vegetative souls possessed by plants, sensitive souls possessed by animals, and rational souls possessed by humans (let’s not get side-tracked by things like the difference between sentience and consciousness). Of course, Aristotle also thought that the Earth was the center of the universe, that some animals spontaneously come into being from mud and earth, and that the heart is the organ of reason and intellect (he believed that the function of the brain is to cool the blood). Those of a religious persuasion will argue that the universe in general — and life in particular — cannot have arisen by chance and must therefore have been designed and created by some intelligent and supernatural entity. By comparison, those of a secular nature take the view that the universe appeared out of nowhere in the form of the Big Bang, that primitive life was kicked off by some random combination of circumstances, and that evolution, as espoused by Charles Darwin, took over, eventually resulting in the pinnacle of life as we know it in the form of me (and you, I suppose, but mostly me). Life is simple (no pun intended) if you assume a deity is in charge. The tricky part for a lot of people who believe in evolution is to explain what happened at the very beginning — how and why (or why and how) did life kick off in the first place? Well, my chum Charles Pfeil just pointed me at an article from the mists of time we used to call 2014 describing a rather interesting theory about why life exists. According to this theory:
[…]when a group of atoms is driven by an external source of energy (like the sun or chemical fuel) and surrounded by a heat bath (like the ocean or atmosphere), it will often gradually restructure itself in order to dissipate increasingly more energy. This could mean that under certain conditions, matter inexorably acquires the key physical attribute associated with life.
I think this article is well worth a read. If true, it would provide the initial link in the chain that grew to be life as we know it. I would view reading this article as a precursor to reading Life’s Ratchet: How Molecular Machines Extract Order from Chaos by Peter M. Hoffmann, which should be followed by Wetware: A Computer in Every Living Cell by Dennis Bray. One last point is that Charles also pointed me to an extremely tasty “Tree of Life” chart on Evogeneao.com. I’ve seen many different representations of the way in which we believe life evolved, but I really do like this one. If you have a moment free, take a look at this video (if you are short of time, start watching at time 3:08).  
I was just thinking back to the question I posed in my Proud to be a Primate column — “Are you a disciple of Intelligent Design, a believer in Flying Spaghetti Monsterism, a devotee of the Invisible Pink Unicorn, or a staunch supporter of Evolution?” As always, I’m interested to hear where you stand (or swing) on all of this.