This Is How Life Began On Earth, Will This All End?

Five billion years ago our planet earth was a very unfriendly place, very hot with carbon dioxide gas bubbled from molten rock and filled the atmosphere, causing such a massive greenhouse effect that the planet literally boiled dry. Living organism could not survive under those conditions. But when water vapour to liquefy just under four billion years ago, life was said to have appeared but was not life as we know it now. Molecules that could replicate to produce daughter molecules with inherited characteristics, eventually microscopic single-celled organisms evolved.

These early life forms had to withstand volatile atmosphere with toxic gases,erupting volcanoes, dramatic electrical storms and the sun’s ultraviolet rays all promoting uncontrolled electrochemical and photochemical reactions. The microbes resembled today’s ‘Extremophiles’, a type of bacteria so-called because they thrive in all the particularly hostile corners of the globe. Extre­mophiles inhabit acid lakes, hyper-saline salt marshes and the super heated water issuing from hot vents at the bottom of the deepest ocean trenches where they survive temperatures up to 1500C and 2500C. They also lie buried deep in the polar ice caps, and lurk in rocks. It is possible that life began with microbes in rocks deer underground, where the heat is intense and there is an ample supply of water and chemicals to get the whole process started.

Coral like structures housed “Extremophiles” (stromatolites), also known as microbial mats because they look like door mats; which are flat, brown and hairy. These have thriving communities of interdependent microbes, utilizing another’s waste to produce energy in a self sustaining food chain or micro-ecosystem. Today, we can still see these in Yellowstone Park, Wyoming, USA and along the shores of Western Australia, where the water is rich in chem­icals and undisturbed by other forms of life. Ancient layered rock structures are the fossilized remains of stromatolites that dominated aquatic ecosys­tems some two billion to four billion years ago.

For around three billion years bacteria had Earth all to them­selves and they diversified to occupy every possible niche. At this stage there was no oxygen in the atmosphere so they evolved many different ways of unlocking the energy bound up in rocks, utilizing chemical compounds of sulphur, nitrogen and iron. Then around 2-3 billion years ago a group of innovative microbes called the cyanobacteria (previously called blue-green algae) learnt the trick of photosynthesis, using sunlight to convert carbon dioxide and water into energy rich carbohydrates. As a result, oxygen, a waste product of this reaction, slowly accumulated in Earth’s atmosphere. At first oxygen was poisonous to early life forms, but then other ingenious bacteria discovered that it could also be used to generate energy. These new energy sources were rich to support more complex life forms, but the emer­gence of multicellular organisms had to await the evolution of eukaryotic cells.

Bacteria are “prokaryotes”, meaning that their cells are smaller than those of all higher organisms “eukaryotes” and have a simpler structure, lacking a well-defined nucleus. But around a billion years ago a group of free-living photosynthetic cyanobacteria took up residence inside other primitive single-celled organisms to form the energy—generating chloroplast of the first plant cells. And in a similarly extraordinary manoeuvre oxygen-utilizing mi­crobes called alpha proteobacteria became incorporated into other microbes as mitochondria, the powerhouse of animal cells.

So finally, a mere 6oo million years ago, the stage was set for the evolution of multicellular organisms made up of eukaryotic cells, and eventually the emergence of the plants and animals we know today. But compared to the diversity of bacteria, all other life forms, however different they may seem, are homogeneous, locked into the same biochemical cycle for energy production, and requiring sunlight for plant photosynthesis to generate the oxygen used by animals for respiration.

We still rely on bacteria (in the form of chloroplasts and mitochondria) for these reactions, and on free-living bacteria for all other chemical processes needed to maintain the stability of the planet. These bacteria recycle the elements which are essential for life on Earth and are at the heart of our balanced ecosystems, those complex interdependent rela­tionships that exist between plants, animals and the environment.

Although bacteria and single-celled protozoa (plasmodium) were the first to inhabit in our earth. Plasmodium that causes malaria, probably represent the earliest and simplest forms of animal life. The tiniest of all microbes, viruses, probably also evolved several million years ago. They have diversified to infect all living things including bacteria, but exactly how and when they came into being is unknown. The genetic material of viruses consists of either DNA or RNA, but most only code for up to aoo proteins and cannot survive on their own. So viruses are obligate parasites and only when they have sabotaged their host’s cells do they spring to life. Once inside they turn the cell into a factory for virus production and within hours thousands of new viruses are ready to infect more cells or seek another host to colonize.

Perhaps because they are so small, nowadays microbes seem to be overshadowed by larger forms of life, but they are still by far the most abundant on the planet, constituting some twenty-five times the total biomass of all animal life. There are well over a million different types, mostly harmless environmental microbes. They are in the air we breathe, the water we drink and the food we eat and when we die they set about deconstructing us. Each ton of soil contains more than 50,000,000,000,000,000 microbes, many of which are employed in breaking down organic material to generate essential nitrates for plants to utilize; every year nitro­gen.-fixing bacteria recycle 140 million tons of atmospheric nitro­gen back into the soil.

Bacteria and viruses are also a key part of marine ecosystems, forming by far the largest biomass in the oceans. There are at least a million bacteria in every millilitre of seawater, most abundant in estuarine waters where they break down organic matter. Marine viruses control the numbers of these bacteria by infecting and killing them, particularly when they undergo a population explosion and produce algal blooms. In coastal waters viruses greatly out­number bacteria, reaching concentrations of around 100 million in every millilitre, totaling an incredible in the oceans. Tiny as they are, if placed end to end thei,- would stretch for to million light years, or too times across the galaxy.

Bacteria are masters at survival, and when adverse conditions come along they are generally ready. Adaptability is the key to their success, yet in theory reproducing by binary fission yields offspring that are all identical to the parent—a process that apparently leaves no room for variability. But although their DNA copying machin­ery is accurate, mistakes occur which are corrected by a cellular proofreading system. Even so, occasional errors slip through un­noticed and these heritable changes to the genetic code (mutations) may cause changes to their offspring. This is the basis of evolution by natural selection. In humans and other animals evolutionary change is a slow process because of our long generation times, but for bacteria, which reproduce very fast and have a less effective DNA proofreading system, rapid change by mutation is their lifeline. A single bacterial gene mutates at a rate of one change per - cell divisions, so in a rapidly dividing colony many thousands of mutants are thrown up. A few of these mutations will confer a survival advantage and these progeny will then quickly out-compete their rivals and come to dominate the population.

Bacteria have several other tricks to help them adapt- rapidly to a changing environment, mostly involving gene swapping. Many bacteria contain plasmids, circular DNA molecules that live inside the bacterial cell but are separate from the chromosome and divide independently. They supply their host bacteria with extra survival information and can pass directly from one bacterium to another during conjugation. This involves the outgrowth of a filament called a ‘sex pilus’ which acts like a temporary bridge between the donor (male) and the neighbouring recipient (female) bacterium giving plasnnds free access and allowing sur­vival genes to spread rapidly through bacterial communities. Several genes that code for antibiotic resistance, allowing bacteria to survive in the face of antibiotic treatment, are carried on plasmids, and they have succeeded in spreading worldwide.

Another way that genes can jump between bacteria is by using viruses called bacteriophages, or phages for short. All viruses are cellular parasites, and phages commandeer the bacteria’s protein making machinery to generate thousands of their own offspring, most of which carry a copy of DNA identical to the parent phage. But around one phage in a million mistakenly picks up an extra piece of DNA, either from the bacterial chromosome or from a resident plasmid, and carries it to the next bacterium it infects, If this extra piece of DNA codes for a protein that improves survival then natural selection will ensure that the offspring of the recipient bacterium will prosper at the expense of others. with their host bacteria, with the phage being safely housed inside the bacterium and the bacterium in turn being protected from infection by other more destructive phages. Remarkably, the toxin that can fatally damage the heart and nerves during a diphtheria infection, and another that causes ‘the catastrophic diarrhoea of cholera, are both coded for by phages resident in the bacteria rather than by the bacteria themselves. Without their phages Corynebacterium diphtheriae and vibriv cholerae are harmless.

At some stage in the distant past, groups of resourceful microbes found a niche in or on the bodies of other living things and evolved to parasitize host species. From that time on the struggle for survival has shaped the evolution of both parties. On occasion, a comfort­able symbiotic relationship developed, like, for example, the mi­crobial communities that form self-sustaining ecosystems in the guts of their hosts. For ruminants such as cows the advantages of this partnership are obvious; the microbes are bathed in nutrients and protected from the outside world while they digest the cellu­lose in plant cell walls which cattle are unable to do for themselves. In humans, however, the function of gut microbes is not so clear. We each house up to 1014 microbes, and out numbering our own body cells by ten to one. So far, more than 400 different species have been identified which probably protect us from attack by more virulent microbes, aid our digestion and stimulate our immunity. They are harmless as long as we are healthy, but if they manage to invade our tissues, perhaps through a surgical wound, they can cause nasty infections.

Of the million or so microbes in existence, only 1,415 are known to cause disease in humans.5 But despite their significance to us, these pathogenic microbes are not primarily concerned with making us ill.

The sometimes devastating symptoms they produce are really just a side—effect of their life cycle being enacted inside our bodies. How­ever, they certainly use each step of the infection process to their own advantage, and natural selection ensures the microbes that induce disease patterns that are best designed to assist their reproduction and spread survive at the expense of their more sluggish siblings. So over time disease patterns have been sharply honed by evolution to ensure the survival of the causative microbes. A highly virulent lifestyle, killing the victim outright, is not advantageous to microbes as they will then be without a home and probably die along with their host. Yet less virulent microbes risk being rapidly conquered by the host’s immune system, and this also curtails their spread. Over centuries of coexistence of microbes and their human host, evolution has fine-tuned the balance between these two extremes to optimize survival of both species, but the rapid adaptability of microbes means that
"they are generally one step ahead in the ongoing struggle, we may never win".
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REFERENCE

"Deadly Companions"
By: Dorothy H Crawford

Delightful, well documented and enlightening. If you are keen to understand more about micro-organism and how they have evolved and learn more about antibiotic resistance, please read this book first.