of Complex Life
What is life?
It is the flash of a firefly in the night.
It is the breath of a buffalo in the wintertime.
It is the little shadow which runs across the grass and loses
itself in the sunset.
Crowfoot, Blackfoot warrior
and orator, 1890
We wish to learn:
- When and how did life arise on earth?
- What kinds of evidence and theory advance our understanding of the origin of life?
- What major evolutionary advances have taken place at the level of the cell?
How life emerged from non-life is an extremely challenging question. As a prelude to the details, let’s review the distinguishing features of living systems in the context of this question. Living systems have three essential properties. (1) They capture energy and utilize energy from external sources to fuel the chemical energy of growth and metabolism. “Autotrophic” organisms synthesize carbon compounds (“food”) with energy from the physical environment, such as sunlight or geothermal heat. “Heterotrophic” organisms procure energy by eating other organisms. (2) Living systems grow by adding materials and by changing organization. (3) Living systems replicate using a structured form of information that passes from generation to generation. This information directs the synthesis and growth of organic compounds in the cell. Non-living systems may exhibit some of these properties but not all of them at once.
Another consideration is the properties of Earth as a planet that are prerequisites for life. These include the size (mass) of Earth, which is large enough to hold an atmosphere by the force of gravity, and the surface temperature of Earth, which supports water in liquid form (fig. 1).
Figure 1. Water on the shore of Lake Superior (in Ontario) flowing over Precambrian rocks. Source: Catherine Badgley.
Part of the challenge in investigating the origins of life on earth is that the rock record from the earliest history of the earth is sparse. Even rarer are sedimentary rocks that have not been weathered away altogether or modified into igneous or metamorphic rocks. Another challenge is that the surface conditions of the early earth differed vastly from those of today, with a higher concentration of greenhouse gases in the early atmosphere, warmer surface temperatures, and anaerobic atmosphere and oceans. Nonetheless, active areas of research focus on plausible hypotheses about when, where, and how life on earth began.
Earliest life: When?
Chemical evidence (carbon isotopes implying photosynthesis) indicates that biological activity was present 3.8 Ga (billion years ago). The earliest fossil cells are known from sedimentary rocks 3.5 Ga in Australia. The fossils depict unicells in spherical and fibrous growth forms clumped together. Such evidence has been contested as the result of inorganic processes, but the co-occurrence of cell-like microstructures along with chemical signatures of organic compounds makes a strong case for these features as fossilized prokaryotes. Given the low probability of preservation of even rocks from this early time period, living cells would likely have appeared even earlier than the oldest fossils. Thus, life on earth probably arose between 4.0 and 3.5 Ga.
Between 3.5 and 2.0 Ga, the most common fossils are stromatolites—microbial mats that grow into small mounds and hummocks by trapping fine mud on sticky surfaces. Figure 2 illustrates modern stromatolites in a tidal setting. Similar microbial ecosystems are still present today in deeper marine and freshwater contexts.
Figure 2. Modern stromatolites. (Source: BBC/Science/Earth)
Earliest life: Where?
1. Ocean. The ideas of Russian scientist Oparin and British scientist Haldane, from early in the 20th Century, postulated that lightning or ultraviolet radiation interacting with the primitive atmosphere would have led to formation of small organic molecules, which would have become locally concentrated in the ancient ocean. According to this hypothesis, clumps of organic molecules would acquire properties of primitive cells, such as semi-permeable outer membranes. Eventually, some of these proto-cells, by a process of selection, became true cells. Coacervates, amoeba-like entities that contain and release compounds, divide, and yet are purely physical in origin, provide a clue to how cell-like properties might have arisen. This hypothesis and later modifications of it have acquired the nickname of “primordial soup.” The environment of early life, under this hypothesis, was the ancient ocean, where early organic molecules accumulated after forming in the atmosphere.
The famous Miller-Urey experiment demonstrated the plausibility of this hypothesis. In the 1950s, Miller and Urey successfully tested the Oparin/Haldane hypothesis. Using a simulated "primitive atmosphere" of methane, ammonia, and hydrogen, and an electric spark, they observed the formation of amino acids in their laboratory apparatus. Further experiments have substituted CO2 for CH4 and NH3, and ultraviolet light for the electric spark and have documented a host of organic molecules in the condensation vessel (fig. 3).
Figure 3. The Miller-Urey experiment. (Source: Wikipedia, Wikimedia)
2. Extraterrestrial locations (fig. 4). The detection of carbon, oxygen, and nitrogen, as well as simple organic compounds, in interstellar gas and dust prompted some scientists to consider whether life originated elsewhere in the solar system and was “seeded” onto Earth via comets and meteorites. Some comets and meteorites contain simple organic compounds, which supports the plausibility of this hypothesis. This hypothesis for the origin of life on earth has fallen out of favor, but it is considered likely that comets and meteorites contributed carbon, oxygen, and nitrogen to the surface of the early earth during the period of intensive bombardment.
Figure 4. The Orion nebula as photographed from the Hubble telescope. Interstellar gas and dust as well as comets and meteorites may contain the common elements of organic compounds as well as small organic molecules. (Source: NASA)
3. Deep earth or submarine hydrothermal vents. The discovery of abundant bacteria deep below the earth’s surface and surrounding submarine vents (fig. 5) prompted some scientists to propose these environments as the locus of the origin of life. Far from damaging ultraviolet radiation and the violent surface processes of the early earth, these deep environments were proposed as potential sites where organic reactions and proto-cells could form. Sulfur compounds and geothermal heat would provide substrates and energy sources for cellular metabolism, as they do for bacteria today in these settings.
Figure 5. Hydrothermal vent. (Source: Woods Hole Oceanographic Institution)
Earliest life: How?
The issue here is whether replication arose first or metabolism arose first in proto-cells. Experiments demonstrating that organic polymers can form on clay-mineral templates or that some forms of RNA can act as a catalyst as well as a means of information transfer have investigated plausible scenarios of replication.
Single origin for life on Earth
Four billion years ago, there may have been many early “experiments” in the formation of organic molecules, proto-cells, and even fully functioning cells. All organisms today, from the smallest microbes to the largest trees and vertebrates, contain a number of biochemical features in common—which supports the idea that all these forms have a common ancestor. Some of these common features are:
- A common set of 20 left-handed amino acids;
- DNA and RNA as the basis of information transfer from generation to generation;
- ATP as the universal currency of energy;
- Fermentation as the first step of carbohydrate metabolism.
Genetic analyses of extant organisms (fig. 6) indicate that life consists of three major branches—the Eubacteria, the Archea, and the Eucarya. Eubacteria and Archae are prokaryotes and Eucarya are eukaryotes. The prokaryotes are the great element cyclers of the earth’s surface, and are fundamental to many ecosystem services. They exhibit astounding metabolic diversity—that is, different kinds can use many different substrates for synthesis and metabolism. Eukaryotes have quite limited metabolic pathways but include all multicellular organisms, span six orders of magnitude of body size, and show great morphological diversity.
Figure 6. The three domains of life as revealed by genetic analyses. (Source: Wikipedia, Wikimedia)
The Eukaryotic Cell
The eukaryotic cell (fig. 7) is larger and more structurally organized than the prokaryotic cell of bacteria and archea. The prokaryotic cell lacks internal membranes; the single-stranded DNA sits in the cytoplasm. In contrast, the eukaryotic cell contains a nucleus and other organelles. The nucleus is surrounded by a membrane, and contains double-stranded DNA in chromosomes. The eukaryotic cell shows other evidence of sub-cellular organization for more efficient function. Various energy-releasing enzymes are organized within mitochondria and plants contain chloroplasts, where photosynthesis occurs.
The origin of the eukaryotic cell over 2 billion years ago was an important evolutionary step. Protozoa, fungi, plants and animals -- are all eukaryotes. The theory of endosymbiosis proposes that at least two organelles found only in eukaryotes -- mitochondria (where ATP, the energy-transfer molecule, is synthesized) and chloroplasts (the location of photosynthesis) originated as prokaryotic hosts engulfed other prokaryotic cells with particular metabolic pathways. This endosymbiotic hypothesis, originally proposed by Lynn Margulis in 1967, provides a mechanism for the evolution of complex cell structures from simpler precursors.
2 nuclear membrane
5 endoplasmic reticulum (rough)
6 Golgi body
8 endoplasmic reticulum (smooth)
Figure 7. Diagram of a eukaryotic animal cell. (Source: Wikipedia, Wikimedia)
Several lines of evidence indicate that life has been present for most of earth history. Hypotheses about life’s origins vary in terms of environmental context and whether metabolism or replication arose first in the earliest cells. Prokaryotic ecosystems dominated the first two billion years of life on earth. Eukaryotes are thought to have arisen from prokaryotic ancestors through stages of endosymbiosis. Our bodies contain ten times as many cells of prokaryotic symbionts and hitchhikers as the number of eukaryotic cells with our human genome.
Ga = billion years ago, Ma = million years ago
The four eons of earth history.
formation of earth and continents, chemical evolution
origin of life, prokaryotes flourish
eukaryotes evolve, development of oxygenated atmosphere, some animal phyla appear
most animal phyla present, diverse algae; explosive evolution of multicellular life forms
- Wilson, E.O. 1992. The Diversity of Life. W.W. Norton and Company, New York.
- Purves, W.K., G.H. Orians and H.C. Heller. Life: The Science of Biology. Sinauer, Sunderland MA.
- BBC Earth Timeline, http://www.bbc.co.uk/science/earth/earth_timeline
Emergence of Complex Life Self Test
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