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.
For reasons noted in the introduction to this chapter, early evidence of life is hard to come by. 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. 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, three kilometres deep off the Galápagos Islands. 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 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.)
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 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).
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 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.
How do we know these things? Specimens of life and associated artifacts have been trapped in muddy sediments, chalk, glacial ice, peat bogs, dry sandy deserts, tree resin etc., for millions of years. These entombments often preserve complete specimens in date-stamped strata for scientists to examine. 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.
- It should be noted that complex (i.e., multicellular) life forms could not have existed anywhere in our universe during the first third or so of its life. It takes several billion years for most stars to burn, then collapse, so producing the novae and supernovae that make and release the heavier chemical elements that partly constitute all planets and life as we know it. It has taken another four billion years for life on this planet to evolve into us. Complex life is a relative late-comer to the universe’s party.
- See Sarah Simpson, “Questioning the Oldest Signs of Life,” Scientific American, April 2003, 70-77, for a recent review of this topic.
- Research carried out by some two hundred scientists from a dozen countries led them to recently state that there are at least five major kingdoms: animals, fungi, green plants, red plants, and brown plants. Their classification is based upon cladistics, a method that groups organisms according to evolutionary characteristics that are genetically shared with a common ancestor. (This contrasts with traditional classification methods whereby life forms are grouped according to the postulated relative importance of shared physical characteristics.)
Undeniably genetic tracing is the more accurate method. However, it is likely that the traditional classification system will continue to be used for many years to come—the scientific nomenclature that has developed over the centuries based upon these approaches is too vast to be revised very quickly.
The modern “family network” (rather than “family tree”) is sketched in the article, “Deciphering the Code of Life” by Francis S. Collins and Karin G. Jegalian (Scientific American, December 1999, 90). It looks markedly different from those traditionally shown in school.
Ian Tattersall, in “Once We Were Not Alone,” Scientific American, January 2000, diagrams (on page 60) the latest thinking about our own (the hominid) family tree. The essay is accompanied by two lovely illustrations of early life painted by Jay H. Matternes. The subsequent issue of this journal (February 2000) outlines the relationships between bacteria, Archaea and eukaryotes. (See in particular W. Ford Doolittle’s article, “Uprooting the Tree of Life,” 90-95.)
- Archaea have now been found to be living in many other environments—animal intestines, compost piles, and marshes, for instance.
- Hydrothermal vents are likely to exist on any planet having a hot core and water. (Possibly most planets possess these two features during their early years, with some retaining them for most of their lives). If so, primitive Archaea-type life forms may be abundant throughout the universe.
- Anaerobic: not requiring air or oxygen. Cyanobacteria still exist and can be found in water and soil, on trees and on rocks. Mats of floating cyanobacteria frequently form mound-like structures called stromatolites. Fossil stromatolites date from all ages, including back to 3.5 billion years ago.
- The transition from prokaryotic to eukaryotic cells is discussed by Christian de Duve in “The Birth of Complex Cells,” Scientific American, April 1996, 50-57.
- Perhaps those that grew larger became more readily visible and excessively preyed upon.
- More than 98% of human genes are identical to those possessed by chimpanzees. (Thus we can effectively resuscitate the “missing links” any time we want—by way of the petri dish and molecular genetic techniques. The recipe would be: take one chimp zygote, replace those DNA portions that differ from ours with human DNA, return to the womb and wait. Turning one human race into another should be even easier: humans are over 99.9% genetically identical.)
- Via Earth’s magnetic field reversal.
- Long before our species appeared, however, the brain pan size of early Homo ancestors began enlarging. This size change, occurring about two million years ago, could be related to the development of language, but, since complex languages probably did not develop until later (see Third-Level Thinking And Language), it is more likely that the increase was a result of the changes introduced by the onset of the ice ages. Having to cope in a frozen environment would have rapidly increased the number of life-threatening problems to be addressed. Larger brain pans in and of themselves do not improve problem solving, but, if the genetic mutation that first brought them into existence also caused an increase in the number of neurons grown, this would. Greater problem-solving ability enhances survival, and the mutated genes that produced a larger brain pan (able to accommodate additional neurons) would have been passed on to subsequent generations.
(Several forces favour smaller heads [not the least being birth canal dimensions] and brain pans stopped enlarging about 200,000 years ago. Possibly word use [and the communal problem solving that third-level thinking and language use encourages] reduced the requirement for further enlargements.)
- The Vostok ice core from Antarctica contains records that date back to 420,000 years ago.
- More than a dozen intriguing photographs of insects entombed in fossilized resin are printed in “Captured in Amber,” by David A. Grimaldi (Scientific American, April 1996, 84–91). DNA from plant and insect life preserved in amber for some 125 million years has been sequenced (i.e., the nucleotide order determined), adding to our understanding of evolution’s pathways.
- See J. William Schopf, Cradle of Life: The Discovery of Earth’s Earliest Fossils (Princeton: Princeton University Press, 1999) for a description of the beginnings and development of the science of precambrian paleobiology.
- Robert Francoeur, Evolving World, Converging Man (New York: Holt, Rinehart & Winston, 1970) provides a nice summary that, in less than twenty pages, describes life’s gradual changes from its beginnings to the rise of man. By letting one day represent a fourteen million year time period, he compresses the more than 3.5 billion years of life’s history on Earth into a one-year time-line. On this scale the Cambrian Period (when most of the major animal groups first appeared) corresponds to November 16–25, and the Jurassic (dinosaur) Age lasts from December 19–22. Early man does not appear until 6:30 p.m. on December 31 (the equivalent of 3 m.y.a. on this one-year time-line). Many similar accounts are in print.