Purpose/2. Life/Development Of Life On Earth
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While we don’t definitively know where life first developed, we do know approximately when it first appeared on Earth—it showed up less than a quarter of a billion years after the Earth’s crust had formed. In other words, just about as soon as it could.[9]
For reasons noted in the introduction to this chapter, early evidence of life is hard to come by.[10] Nevertheless, indirect evidence suggests that it was present at least 3.7 billion years ago. This has been deduced from an analysis of rocks dating to that age, found on an island close to Greenland. These rocks contain a higher carbon-12 to carbon-13 isotopic ratio than chemical and physical processes alone would create. (Life processes prefer the lighter isotopes, and this concentrates carbon-12 where life exists.) More direct evidence, in the form of fossil micro-organisms, has been discovered in sedimentary rocks from Iceland that are between 3.7 and 3.8 billion years old. (Iceland is particularly suitable for finding early life forms because its rocks have not been greatly disturbed by geological processes during the intervening ages.)
Many of us were taught in school that there are three kingdoms of life on this planet.[11] The simplest and most ancient are called the Archaea (otherwise known as archaebacteria, the first cells).
Archaean kingdom representatives were first discovered in volcanic vents on the floor of the Pacific ocean,[12] three kilometres deep off the Galápagos Islands.[13] Archaea and very primitive bacteria are autotrophic (that is, they build their complex living molecules by chemosynthesis, a chemical process mentioned in the previous section).
The second kingdom, the Prokarya, are a later development; they consist of life forms whose cells lack internal membranes (and thus have no nuclei).
Prokaryotic life was flourishing within the Earth’s shallow oceans as blue-green algae (a.k.a. cyanobacteria), over three and a half billion years ago. Once formed, the anaerobic[14] cyanobacteria began dumping its photosynthetic by-product, oxygen, into the Earth’s oceans and atmosphere, and continued to do so without much competition for over two billion years. Eventually a new form of bacteria evolved that was able to use this oxygen through a process we call aerobic respiration; this opened the way for the more complex (and more energy-demanding), nucleated, eukaryotic cells to evolve. (Bacteria, of course, still exist in abundance everywhere conditions permit, and they still lack cell nuclei.[15])
The third kingdom, the Eukarya, first appeared about two billion years ago. The cells of eukaryotic life forms contain membrane-bound nuclei, and all plants and animals (including humans) belong to this kingdom.
While one billion years ago the continents were still barren (with the possible exception of primitive algae), the seas teemed with unicellular life. Many of these life forms reproduced asexually through division, although some used sexual means. About 700 m.y.a. (million years ago), multicellular sea plants appeared. They rapidly developed in form and prevalence as they made the most of their added capabilities. Multicellular sea plants stayed at the forefront of life’s evolution until the beginning of the Cambrian era, about 540 m.y.a., when multiple forms of marine animals developed from simpler varieties of roundworms. This transition occurred because possession of body cavities and an alimentary canal allowed worm-like creatures to grow more than a few cells thick (as nutrients and waste materials could now be readily passed between internal cells and the external environment). Larger bodies meant that supporting structures would be valuable adaptations, and any that evolved would be retained. The first vertebrates developed soon thereafter (about 500 m.y.a.).
By 400 m.y.a., plants, fungi and primitive arthropods (invertebrates, similar to crabs or lobsters, having an external skeleton and jointed appendages) had colonized the ocean shores and moved inland. (The ongoing evolution of early arthropods eventually produced spiders, centipedes and insects.) Around this time, fish utilized their swim bladders and fins to spend temporary periods on land. These organs gradually evolved into lungs and legs, and the animal class known as amphibians arose. The fluid-filled amniotic sacs we call eggs allowed amphibians to reproduce and give birth on dry land, and some later evolved into reptiles, dinosaurs, lizards, snakes and turtles.
The earliest mammals appeared some 200 m.y.a., evolving from a group of reptiles called therapsids. These mammals were small (about five centimetres long) and possibly lived in trees during the dinosaur age. They remained rodent-like creatures[16] until the dinosaurs became extinct 65 m.y.a. One branch of these early mammals evolved (some 30 m.y.a.) to become Proconsul, our hominoid ape ancestor, and their descendants became the gibbons, orang-utans, gorillas and chimpanzees we know today. About six million years ago, the ape and hominid lineages separated; today our closest living relatives are Central African chimpanzees (demonstrated and verified by comparative DNA sequencing[17]).
The genus Homo appeared about two and a half m.y.a. (although stone tools have been found that date to earlier periods). Artifacts left by “technologically advanced” clans of early humans (who used stone tools to chip bones and antlers into refined shapes) have been found in Israel’s Dead Sea Rift Valley and dated definitively[18] to 780,000 years ago.
Neandertals (who first appeared in Europe about 200,000 y.a. and whose ancestors were hominids who moved from Africa to Europe some 500,000 y.a.) holed up in valleys to survive the ice ages and so avoided the many challenges that constant moves would have brought. Perhaps as a result, their tools changed little during most of their existence, and this suggests that their intelligence also did not greatly change. However, fossilized bone structures show that Neandertals did have the means to utter words, and they probably developed and used simple languages.
The tools and ornaments of Homo sapiens, on the other hand, changed greatly over very short periods of time. Our species first appeared in Africa over 100,000 y.a. and moved into Europe (as Cro-Magnon) around 40,000 y.a., and they seemed to have confronted and surmounted the various challenges successive ice ages introduced.[19]
How do we know these things? Specimens of life and associated artifacts have been trapped in muddy sediments, chalk, glacial ice,[20] peat bogs, dry sandy deserts, tree resin[21] etc., for millions of years. These entombments often preserve complete specimens in date-stamped strata for scientists to examine.[22] Painstaking observations over many decades combined with more recent sophisticated analytical techniques (such as DNA analysis and various imaging techniques) consistently show that life’s development demonstrates an overall progressive trend from simple to complex.[23]
