Solar System Neighbours
Life on Earth consists of millions of species, including all forms of bacteria, fungi to plants and animals. There are forms of life populating every possible environment on Earth. From the ocean depths, where many strange creatures live, to the Earth’s atmosphere where there are microorganisms floating about. There are algae that can live below zero temperatures under ice sheets, and there are bacteria that can live at near boiling temperatures in hot springs .
So, life has been highly successful on this planet, filling every possible niche, and yet close by, our nearest solar system neighbour, the moon, is entirely devoid of life. What about the rest of the solar system?
While efforts are being made to find possible life on Mars, a planet close to us, there is still no direct proof of life there either. Mars is quite a bit smaller than Earth, is significantly colder and lacks an atmosphere capable of sustaining life as we know it. Yet there could still have been non oxygen–dependent life there. From extensive data sent by the probes roaming the surface of Mars, there is no life so far, and this is despite clear evidence that flowing water has been present on Mars at some time in its history.
Venus, another close solar system neighbour, has an atmosphere so thick and hot that it would melt lead, and this condition, alone rules out any thought of life taking hold there. Even if there was life once, it would have been long obliterated by the current highly acidic and very hot atmospheric conditions there. The atmosphere is rich in carbon dioxide, affording a runaway greenhouse effect and heating the atmosphere to very high temperatures.
Other planets are either too close to the sun as Mercury, or too far away and therefore too cold as in Jupiter, Saturn, Uranus and Neptune. Some moons of these planets are thought to contain some liquid water under their icy surfaces, such as Jupiter’s Europa and possibly Saturn’s Enceladus.
Why Does Earth Support Life while other Planets in our Solar System Do Not? The Goldilocks Principle
Earth is home to life because it is just the right distance from the sun. By virtue of having an atmosphere containing gases, including carbon dioxide, nitrogen and water vapour, enough solar heat is trapped to allow water to exist in mostly its liquid state. Earth’s atmosphere also contains about 21% oxygen which generates ozone in the upper layers and this protects life on Earth from exposure to damaging levels of ultraviolet radiation. The warming provided by the other gases provides and maintains a narrow range of temperatures for life to exist. This is a level of global warming, known as the greenhouse effect, that is necessary for life, as opposed to the human-created artificial high levels now threatening life on Earth by enhancing the level of warming at too fast a rate for biological systems to adapt. Without the level of climate moderation that the atmosphere provides, Earth would be too hot during the day and too cold during the night for life to exist. At the same distance from the sun as the Earth, with no atmosphere, the moon adequately demonstrates this.
The circumstances of Earth’s position in the solar system, and the contribution made by its atmosphere, which gives rise to the ideal life-supporting conditions, has been called the Goldilocks principle. Earth is not too hot, nor too cold, but just right for life. It is a subject of philosophical argument whether this principle is a product of our human desire to fit everything in nature into our (anthropocentric) world view or whether there is some other, yet undiscovered, process at work which results in these ideal conditions; or is it just a set of chance happenings?
Life on Earth, Running the Program Backwards
What Charles Darwin and Alfred Wallace showed in their theory of Evolution, which they arrived at independently, was that all the life forms on Earth at the present time have evolved from previous life forms . This is now one of the most scientifically supported theories that science has come up with, and it is a guiding principle for all biological studies. Ultimately the strongest evidence supporting the theory is best found by looking at DNA encoding genes from all species. There are genes encoding for many important proteins which are nearly identical (highly conserved amino acid sequences) across all species. Indeed, the mechanism by which the genes are translated into proteins, the Genetic Code, is close to identical for all species on Earth (some very primitive species have slight variations in the code-a clue for its origin!) Therefore, knowing how identical for all species is the process by which DNA is replicated (copied) and passed on to new cells, and how identical is the process by which genes for all species are translated into proteins, there can be no other credible origin for evolution than that all species on Earth have arisen from a common ancestor.
If we run the program backwards where those life forms arose, we find that by looking at DNA sequences, humans’ closest relative is the chimpanzee but, to correct a common misconception, the chimpanzee is not a species from which humans arose. Humans, bonobos (another chimpanzee-like species) and chimpanzees had a common ancestor at one time from which we all diverged only as recently (in geological terms) as a few million years ago. Perhaps this divergence occurred more recently, since the oldest known identifiably human fossils found in Morocco recently are dated at 300,000 years old. The human/chimpanzee/bonobo ancestor no longer exists, but it in turn had an ancestor, and if we wind the tape backwards through possible precursor ape species we arrive at a lemur-like mammal that gave rise to the apes and all that family of mammals.
Richard Dawkins has explored these lineages exhaustively and illustrated them well in a book called ‘The Ancestor’s Tale’ . Going further back, we encounter a species that developed fur and a warm-blooded metabolism to split from its cold blooded dinosaur–like reptilian ancestors. It was a mammalian species that survived the asteroid impact in the Yucatan peninsula which the dinosaurs did not survive. Reptile lineage can be traced back through their antecedents, the amphibians, the first creatures to have both an aquatic and a land based existence. Their descendants today are frogs and salamanders. Prior to amphibians were lungfish, which of course, evolved from fish. There are still lungfish in waters today and these can be considered living fossils in that they have examples in the fossil record as well as living examples today. The most archetypical of these living fossils is the coelacanth, rediscovered off the east coast of Africa in 1938, not so long ago.
The species which came before fish were arthropods such as crustaceans, crabs and a vast number of insect species and their precursor species are creatures like trilobites of which only fossil evidence remains.
Mass extinctions: A fascinating study of the earliest arthropods can be found in looking at Precambrian fauna. Fossils of this era were first found in a deposit called the Burgess Shales at Yoho National Park in Alberta, Canada. The story of the Burgess Shale fossils and their significance were illustrated in a book called “Wonderful Life” by Stephen Jay Gould . In this absorbing book some interesting observations are made about the variety of different body plans, most with exoskeletons (such as found in crabs and lobsters), that once existed and how they were largely obliterated by a mass extinction event. Probably an asteroid impact, the evidence for which is still controversial. It is likely that the remaining dominant species after that event were the trilobites. Gould points out that there is no way to predict which species will survive such a mass extinction event, but the end result is that all species that follow are derived from those remaining species. This means that all body plans are reduced to being derived from the ones remaining which are contained in those surviving species. That might be only a few percent of the variety that existed before the mass extinction event.
Mass extinction events have happened a number of times in Earth’s prehistory, and each time, the species that are left will diverge to fill all the ecological niches left vacant by the mass extinctions.
Fossils of the Earliest Life
Predating these fossils with exoskeletons were a group of more primitive worm-like creatures in a fossil deposit in South Australia in the Ediacaran area of the Flinders Ranges. Discovered by Reg Sprigg, these fossils from 635 to 542 million years ago, of simple organisms with very primitive body plans, which had virtually no hard parts or exoskeletons, are some of the earliest known complex organisms (e.g., Fig. 1) The South Australian museum houses a very good collection of these fossils.
Fig. (1) Dickinsonia, Ediacaran fauna.
Deposits of Ediacaran fauna have now been found in other parts of the world, including China. The simplest and earliest known fossil life forms are again found in Australia and Canada and are formed from mats of cyanobacteria. There are fossil forms of these mats which are known as stromatolites (Fig. 2). The reason we can be fairly sure that the fossils are of stromatolites is that there are living examples of stromatolite mats in Western Australia at a place called Shark bay (Fig. 3) and recently, some have been found living in the waters of an extinct volcano caldera called the Blue Lake at Mt Gambier in South Australia.
Fig. (2) The cyano bacteria found in stromatolites are dated to a time prior to the advent of atmospheric oxygen.
Fig. (3) Living stromatolites.
Many fossil stromatolites can be found throughout the world, including Canada and Australia. The oldest known are from an area called the Warrawoona formation in a very hot and dry place, ironically called the North Pole, in Western Australia. They have been dated at 3.47 billion years old. This, and the methods used to date the rocks, is detailed in a book called “Life on a Young Planet” by Andrew Knoll . Fossils of possible bacteria from times earlier than 3.47 billion years are the subject of fierce debate among paleontologists and suffice it to say the jury is still out on these specimens. Knoll develops these arguments in his book mentioned above.
Bacteria and Archaea
It is not worth re-running the tape back through all the species exhaustively for this exercise (Dawkins has already done it in the Ancestor’s Tale), but it should be enough to know that all species have arisen from previous species through an unbroken line of descent. We can go back through dinosaurs, amphibians, fish and worms to ultimately look at algae and other small multicelled creatures but these in turn have developed from single-celled organisms.
Ultimately the single-celled organisms, which we know of as bacteria, would have evolved from a precursor organism which is no longer present. Such is the pace at which bacteria evolve which can be seen by the rate that they adapt to entirely new food sources or other challenges. Therefore it is hard to determine which path their evolution may have taken. However, there is a very strong clue in the existence of another completely separate class of single celled organisms known as the archaea.
Now classed as a separate family from the bacteria and from higher organisms, archaea have a set of unique chemical components which set them apart from bacteria. They are found in highly unusual environments and for that, they are also known as extremophiles. For example, they can be found in huge numbers in soil, in hot springs, in highly acidic water and highly saline water. The features of these organisms that allow them to withstand these extreme conditions are not well understood but include, for example, the use of internal structures made of protein that resist decomposition by heat. A unique feature of archaea is the composition of their cell membranes which differ from bacterial cell walls in being composed of isoprenoid structures as opposed to the fatty acyl triglyceride structures of bacteria and eukaryotes. In the triglyceride molecules of most modern bacterial cells, fluidity of membranes is conferred by adding double bonds in the fatty acid structure. This is called a level of unsaturation and the greater the level of unsaturation the greater the membrane fluidity. In archaea, the fluidity of their membranes is conferred by virtue of the branched chain structure of the isoprenoids. This will become important when we look at vesicle formation.
Oxygen and Complex Organisms
Stromatolite or actually cyanobacterial photosynthesis would have contributed significantly to the production of atmospheric oxygen. The arrival of oxygen in turn allowed the development of multicellular organisms. Diffusion of oxygen through several cell layers made possible the addition of more than one layer of cells. Oxygen allowed organisms to develop a new way to derive more energy from their food. Respiration allowed organisms to make use of more energy from their carbohydrate food source by more efficiently oxidising the food completely to carbon dioxide. In doing so, animals were able to make the most efficient use of the oxidisation process. Up until the advent of oxygen in the atmosphere, organisms could only use anaerobic (non-oxygen-dependent) metabolism to derive energy. This is akin to the process of fermentation in that it gave rise to a product such as lactic acid which contained three carbon atoms but could be utilized no further. Anaerobic metabolism of a glucose molecule could only reach a total of 140kcal/mole of energy from the process of glycolysis and in doing so, its end product, pyruvic acid, was a three carbon species which could convert easily into ethanol or lactic acid.
Aerobic metabolism, (respiration), took those three extra carbon atoms all the way through to carbon dioxide by a cyclic process known in modern biochemistry as the citric acid cycle, also known after its discoverer, Sir Hans Krebs, as the Krebs cycle. (Krebs recounted, in a lecture I attended, that his paper was rejected by several journals because the concept of a metabolic cycle was too radical at the time to be believed. It was, of course, ultimately accepted.) There are six carbon atoms altogether to be oxidized since six carbon glucose is split into two three carbon units during the anaerobic stage glycolysis process. This process allowed the amount of energy to be derived from one glucose molecule to reach 546 kcal/mole. The theoretical maximum amount of available energy from the process is 686 kcal/mole so the glycolysis -citric acid cycle oxidation of glucose is 79.6% efficient (Fig. 4). Energy from a simple burning of glucose would only reach around 15-20% in most machines using combustion of carbon as the energy source.
Fig. (4) Glycolysis and citric acid cycle.
The energy released from this controlled stepwise oxidation process is stored in a molecule called adenosine triphosphate or ATP, a phosphate rich molecule with a sugar backbone and a nucleotide handle. ATP is the energy currency of organisms and it is used in the body to power everything from the synthesis of new cells to the energy required for muscle contraction. All of the above functions are necessary for the maintenance of life in complex organisms.
Early anaerobic life, however, made use of only the glycolysis part of the energy release. It was not until the advent of cyanobacteria which could trap atmospheric carbon dioxide and release oxygen as a waste product (photosynthesis) that the development of complex life, which could utilise oxygen in the aerobic steps, became possible. The release of oxygen into the atmosphere also created a huge amount of oxidized iron which can be found as a mineral, iron ore.
Last Universal Common Ancestor
Having outlined some of the latter steps leading to complex life and on, up to mammals, plants and ultimately humans, it is more challenging now to go back further and look at the possible steps leading to life in its simplest forms. If we go back and look at the single celled species of bacteria and archaea it becomes clear that both arose from a single precursor life form because both families have essentially the same genetic makeup. An organism that gave birth to both lineages has therefore been proposed and it is called the last universal common ancestor (LUCA).
What ingredients and conditions do we need to create LUCA?
Carbon, nitrogen, oxygen, sulphur, phosphorus, iron.
Membrane vesicles or fatty acids that make them
Amino acids, and/or other organics
Template system for reproduction
Condensation conditions to allow the formation of polymers such as polypeptides
An energy generating system, probably iron/sulphur or perhaps a proton gradient.
Where might we go looking for these ingredients and conditions?
Before doing that let us explore in chapter one the conditions that existed on a newly formed Earth and how anything could live on our newly formed planet?
The author wishes to thank emeritus A/Prof Victor Gostin of the geology Department of the University of Adelaide for useful discussions. The author wishes to thank Dr. Jasmine Evergreen for beta reading the manuscript and making useful observations.