Figure 11.23
Stromatolites are microbial mats made by some of the earliest photosynthetic organisms on Earth. Fossil stromatolites (left) are among the oldest fossils on Earth, although some have been interpreted to be of abiotic origin. Living stromatolites (right), mats of cyanobacteria, are found primarily in hypersaline lakes and marine lagoons.
Oxygen produced by photosynthesis first oxidized iron dissolved in the oceans, creating massive deposits of iron ore. Eventually, toward the end of the Archean, oxygen began to accumulate in the atmosphere, creating a major environmental change that is sometimes called the “Oxygen Catastrophe.” Oxygen was indeed toxic to many of the prokaryotes which had evolved as anaerobes. However, ultraviolet rays converted some of the oxygen to ozone, which prevented much of that harmful radiation from reaching the earth’s surface. Thus, while an oxygen atmosphere may have killed many species, it allowed survivors to colonize previously uninhabitable ocean surface and terrestrial habitats. Even more important to the future of life, some prokaryote survivors “learned” how to use oxygen to harvest a great deal more energy from organic molecules. The energy efficiency of aerobic respiration paved the way for the emergence of larger and more complex organisms in the Proterozoic Eon.
Eukaryotes: Alliance, Invasion, or Slavery?
You have learned that our own eukaryotic cells protect DNA in chromosomes with a nuclear membrane, make ATP with mitochondria, move with flagella (in the case of sperm cells), and feed on cells which make our food with chloroplasts. All multicellular organisms and the unicellular Protists share this cellular intricacy. Bacterial (prokaryotic) cells are orders of magnitude smaller and have none of this complexity. What quantum leap in evolution created this vast chasm of difference?
The widely accepted Endosymbiotic Theory, shown in Figure below, proposes that many organelles were once independently living cells. Larger cells engulfed these smaller cells but did not digest them, perhaps due to prey defenses. Alternatively, perhaps the smaller cells invaded the larger cells with the “intent” to parasitize. In either case, with their own DNA, the endosymbionts reproduced independently within the cell, and cell division passed them on to future generations of cells. Aerobic bacterial invaders would have been able to use oxygen to further break down and use energy from the host’s “wastes” from glycolysis. So much energy (ATP) resulted that some was available to the host; a mutually beneficial symbiosis resulted. This intriguing story of cooperation – so different from natural selection’s emphasis on competition – explains the origin of our mitochondria. A similar tale is told for chloroplasts; the benefit for a heterotrophic “host” is clear. Some scientists view cilia, flagella, peroxisomes, and even the cell nucleus as endosymbionts, but these ideas are less widely accepted.
Figure 11.24
The Endosymbiotic Theory holds that eukaryotic cells arose when larger prokaryotic cells engulfed smaller, specialized prokaryotes, without later digestion. The smaller cells reproduce independently within the larger cells, to the potential benefit of both. The diagram shows possible events leading to endosymbiosis. Black: membrane; Pink: eukaryotic DNA; Green: cyanobacteria/chloroplast DNA; Red: proteobacteria/mitochondrial DNA
What is the evidence for this maverick evolutionary pathway? Biochemistry and electron microscopy provide convincing support for the Endosymbiotic Theory. The mitochondria and chloroplasts which live within our eukaryotic cells share the following features with prokaryotic cells:
Organelle DNA is short and circular – and sequences do not match DNA in the nucleus.
Molecules that make up organelle membranes resemble those in prokaryotic membranes – and differ from those in eukaryotic membranes.
Ribosomes in these organelles are similar to those of bacteria – and different from eukaryotic ribosomes.
Reproduction is by binary fission – not mitosis.
Biochemical pathways and structure show closer relationships to prokaryotes.
Two or more membranes surround these organelles.
The “host” cell membrane and biochemistry are more similar to those of Archaebacteria, so scientists believe eukaryotes descended more directly from that major group (Figure below). However, the standard evolutionary tree cannot accurately depict our ancestry, because the origin of the eukaryotes combines traditional descent from the Archaea with landmark cohabitation alliances forged with the Bacteria.
The timing of this dramatic evolutionary event (more likely a series of events) is not clear. The oldest fossil clearly related to modern eukaryotes is a red alga dating back to 1.2 billion years ago. However, many scientists place the appearance of eukaryotic cells at about 2 billion years. Some time within Proterozoic Eon, then, all three major groups of life – Bacteria, Archaea, and Eukaryotes – became well established. Geologists hypothesize the oldest supercontinent, Columbia, between 1.8 and 1.5 years ago, as the backdrop for the further evolution of these three domains.
Eukaryotic cells, made possible by endosymbiosis, were powerful and efficient. That power and efficiency gave them the potential to evolve new ideas: multicellularity, cell specialization, and large size. They were the key to the spectacular diversity of animals, plants, and fungi which populate our world today. We will tell their much more familiar story in the next lesson. Nevertheless, as we close the history of early life, reflect once more on the remarkable but often unsung patterns and processes of early evolution. Our “size-ism” sets us up to wonder at plants and animals, and ignore bacteria. Our human senses cannot directly perceive the unimaginable variety of single cells, the architecture of organic molecules, or the intricacy of biochemical pathways. Let your study of early evolution give you a new perspective – a window into the beauty and diversity of unseen worlds – now and throughout Earth’s history. Apart from the innumerable mitochondria which call your 100 trillion cells home, your body contains more bacterial cells than human cells. You, mitochondria, and your resident bacteria share common ancestry – a continuous history of the gift of life.
Figure 11.25
The three major domains of life had evolved by 1.5 billion years ago. Biochemical similarities show that we Eukaryotes share more recent common ancestors with the Archaea, but our organelles probably descended from Bacteria by Endosymbiosis.
Lesson Summary
The Big Bang and stars such as red giants made the atoms which build life.
Earth gradually condensed into a molten protoplanet, constantly bombarded with debris.
Steam escaping from the formative crust and volcanic gases contributed to Earth’s early atmosphere, which probably contained methane, ammonia, carbon dioxide, water and nitrogen. Such an atmosphere would be toxic to most modern organisms.
No oxygen meant no ozone; ultraviolet radiation reached the Earth and threatened life with deadly, mutating rays.
Eventually, water in the atmosphere condensed into clouds and rain, forming oceans.
Earth’s oldest known rocks are between 3.8 and 4.2 billion years old. The oldest minerals are 4.4 billion years old. Scientists estimate that the age of the Earth is 4.6 billion years.
Miller and Urey showed that a spark igniting a mixture of gases resembling Earth’s primitive atmosphere could produce most of the building block organic molecules of life – forming an “organic soup.”
Some lipids and certain polypeptides can spontaneously form into protocells; early membranes could have self-organized in this way.
The “Genes-first” hypothesis proposes that replicating molecules evolved before biochemical pathways.
Some scientists believe RNA, rather than DNA, was the first replicator.
The “metabolism-first” model suggests that biochemical pathways evolved in an organic soup before self-replicating molecules.
Many scientists accept that a “last universal common ancestor” (LUCA) cell arose from the primeval soup of organic molecules.
This prokaryote probably had a cell membrane and ribosomes, and used DNA for informatio
n storage, RNA for information transfer, and protein for catalyzing chemical reactions – like all life today.
The first cells were probably heterotrophs feeding on organic soup, or chemoautotrophs using the energy in inorganic molecules.
Not long after the LUCA prokaryote arose, life split into two groups, Bacteria and the Archaebacteria.
Photosynthesis arose roughly 3 billion years ago.
The oldest fossils, stromatolites, preserve photosynthetic cyanobacteria.
Oxygen produced by photosynthesis eventually changed Earth’s atmosphere.
Ozone formed, protecting life from damaging UV radiation.
The widely accepted Endosymbiotic Theory explains the origin of eukaryotic cells as a merging of several kinds of prokaryotic cells.
Review Questions
Why is understanding the nature of science important to studying the origin of life on Earth?
Interpret the statement “we are made of stardust.”
Describe the effects of the moon on the conditions for life on Earth, according to the impact theory of the moon’s origin.
Discuss the formation of Earth’s atmosphere and compare it to today’s.
Identify the age of the Earth, and give the supporting evidence.
Describe Miller and Urey’s experiment, and evaluate its importance to our understanding of the origin of life.
Compare and contrast the RNA World, genes first, metabolism first, and exogenesis models of the origin of life. Evaluate the evidence supporting each model.
List the characteristics scientists attribute to “last universal common ancestor” of life on Earth.
Indicate when scientists believe photosynthesis originated, and what evidence suggests this. Analyze the effects of the origin of photosynthesis on life existing at that time.
Analyze the theory which explains our current understanding of the origin of eukaryotic cells. In what way does it differ significantly from “traditional” ideas of evolution?
Further Reading / Supplemental Links
Mark Pagel, ed. The Oxford Encyclopedia of Evolution. New York: Oxford University Press, 2002.
Stephen Jay Gould, ed. The Book of Life: An Illustrated History of the Evolution of Life on Earth. New York: W.W. Norton, 1993.
Lynn Margulis et al. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth. New York: W.H. Freeman, 1998.
Cain et al., “Endosymbiosis Animated.” From Discover Biology, Third Edition, W. W. Norton & Co.© 2006 W. W. Norton & Co. and Sumanas, Inc.Available on the Web at:
http://www.sumanasinc.com/webcontent/anisamples/nonmajorsbiology/organelles.html
Dave Smith, “Life Has a History – Level 2.” University of California Museum of Paleontology, 7/18/06. Available on the Web at:
http://www.ucmp.berkeley.edu/education/explorations/tours/intro/Intro5to12/tour1nav.php
Lexi Krock, “The Missing Link: A Brief History of Life.” Nova Online, last updated February 2002. Available on the Web at:
http://www.pbs.org/wgbh/nova/link/history.html
Richard Cowen, “History of Life, 4th edition Updates, References and weblinks.” UC Davis Geology Department, 3 March 2006. Available on the Web at:
http://www-geology.ucdavis.edu/~cowen/HistoryofLife/
Roger Perkins, “The Virtual Fossil Museum: Fossils across Geologic Time and Evolution.” Available on the web at:
http://www.fossilmuseum.net/index.htm
Roy Caldwell and David R. Lindberg, “Evolution 101.” University of California Museum of Paleontology, 2007. Available on the Web at:
http://evolution.berkeley.edu/evolibrary/article/evo_01
Roy Caldwell and David Lindberg, “Understanding Evolution.” University of California Museum of Paleontology, 2007. Available on the Web at:
http://evolution.berkeley.edu/
Vocabulary
endosymbiotic theory
Theory which proposes that many organelles were once independently living cells; describes the formation of eukaryotic cells.
genes-first model
The idea that a successful replicator molecule preceded the evolution of biochemical pathways.
LUCA
The last universal common ancestor; the first true cell, which formed about 3.5 billion years ago.
metabolism-first model
The proposal that extensive evolution of biochemical pathways might have preceded replicator molecules and individualization of life.
organic molecules
The “materials of life” - molecules made primarily of the element carbon.
oxygen catastrophe
Toward the end of the Archean, oxygen began to accumulate in the atmosphere, killing many anaerobic species.
primeval soup
Oceans in which gradual chemical evolution formed life; proposed by Aleksandr Oparin.
protocells
Simple, membrane enclosed early metabolic units surrounded by phospholipids or polypeptides; precursors to true cells.
RNA world hypothesis
Hypothesis that proposes that RNA evolved prior to DNA.
Points to Consider
Which theory of life’s origins do you consider most plausible: genes first, metabolism first, or exogenesis? What kinds of evidence would be required to support each theory?
The standard form for an evolutionary tree is a series of branching lines which show common ancestors. Can you imagine a format which could show Endosymbiosis, as well as common ancestry?
Lesson 11.3: Multicellular Life
Lesson Objectives
Assess the impact of global environmental changes on the evolution of life.
Describe the diversity of unicellular organisms which arose over 2 billion years of evolution.
Evaluate the importance of major evolutionary developments which preceded the Cambrian explosion: colony formation, cell specialization, and sexual reproduction.
Evaluate the importance of some factors which contributed to the “Cambrian explosion” of biodiversity.
Trace the evolution of plants and animals from aquatic to terrestrial habitats.
Connect changes in atmospheric O2 and CO2, temperature, geography, and sea level to extinctions and radiations of various groups throughout the Paleozoic.
Identify recurrent extinctions as losses of diversity, but also opportunities for the evolution of new species.
Describe the conditions under which the dinosaurs emerged to dominate life on Earth.
Identify the diversity of habitats and niches occupied by the dinosaurs during their “golden age.”
Discuss the relationships between reptiles, birds and mammals during the age of the dinosaurs.
Explain the coevolution of flowering plants and insects during the Cretaceous.
Evaluate the evidence for an “impact event” as the primary cause of the K-T extinction which ended the reign of the dinosaurs.
Analyze the emergence of mammals and birds as the dominant land animals during the early years of the Cenozoic.
Connect sea level, land bridges, and climate to their effects on evolution.
Explain the connection between CO2 levels, temperature, and glaciation.
Discuss the factors which contribute to the “sixth” major extinction.
Introduction
Biologists estimate that 99% of the species which have ever lived on Earth are now extinct, and up to 80 million species populate our world today. It is the great diversity of species that allows at least some organisms to survive major changes in the environment.
4 billion years of simple, prokaryotic cells
3 billion years of photosynthesis
2 billion years of complex, eukaryotic (but still single!) cells
1 billion years of multicellular life
Figure 11.26
The history of life reaches the last billion years of Earth’s 4.6 billion-year history with no hint of the wondrous diversity of life as humans know it. Not until nearly 80% of
Earth’s history had passed did multicellular life evolve. The fossil record tells the story: millions of species of fish, amphibians, reptiles, birds, mammals, mosses, ferns, conifers, flowering plants, and fungi populated the seas and covered the Earth - as continents crashed together and broke apart, glaciers advanced and retreated, and meteors struck, causing massive extinctions. Life has had a colorful and exciting last billion years, spawning diversity almost beyond our comprehension.
And yet, the giant steps of evolution remain back in the Precambrian. Its catalog of evolutionary innovations is long and impressive:
Energized elements from stardust formed simple organic molecules.
Building blocks chained together to form catalysts and self-replicating macromolecules.
Biochemical pathways evolved.
Protective yet permeable membranes enclosed the catalysts, replicators and their metabolic retinue.
Early prokaryotic cells “learned” to make ATP by splitting glucose.
Others began to harvest sunlight energy through photosynthesis.
Photosynthetic cyanobacteria produced vast amounts of “waste” oxygen, dramatically altering the Earth’s atmosphere.
The oceans rusted (iron ore deposits).
An ozone layer formed, shielding life from UV radiation.
The “O2 catastrophe” killed many anaerobic prokaryotes.
Still other prokaryotes “learned” to use the new O2 to release the energy remaining in carbohydrates products of glycolysis.
CK-12 Biology I - Honors Page 48