Vocabulary
absolute aging
Measures half-lives of radioactive isotopes to calculate the number of years which have elapsed since a rock formed; also known as absolute dating.
adaptive radiation
A pattern of speciation which involves the relatively rapid evolution from a single species to several species to fill a diversity of available ecological niches.
coevolution
Evolution in which two species or groups of species influence each other’s evolution and therefore evolve in tandem.
coextinction
If one member of a pair of interdependent species becomes extinct, the other is likely to become extinct as well.
convergent evolution
Evolution whereby distantly related species evolving similar traits.
divergent evolution
Evolution whereby closely related species evolve different traits.
eons
The largest units of time within the Geologic Time Scale; divided into eras, which are also divided into periods, epochs, and stages.
episodic speciation
A pattern of periodic increase in new species; follows mass extinctions as well following major new “discoveries” or “ideas” – for example, the biochemical pathways for photosynthesis or cellular respiration.
extinction
The death of an entire species.
fossils
The preserved remains or traces of organisms; provide extremely rare but vivid windows to the past.
Geologic Time Scale
A column of rock layers which reflects the history of sedimentary rock formation and changing life.
gradualism
The idea that evolution progresses via slow, steady, gradual change; suggests that changes accumulate continuously as one species evolves to become another.
index fossils
Widespread, short lived fossils that can be used to help identify rock layers of the same age spread around the Earth, providing the relative age of other fossils.
macroevolution
Evolution at or above the species level.
microevolution
Describes changes within a species or population.
molecular clocks
Measure changes in DNA or proteins to indicate degrees of relationship among species.
paleontologists
Scientists who study fossils.
punctuated equilibrium
Proposes that species remain the same for long periods, and that change occurs infrequently but rather rapidly under unusual conditions such as geographic isolation or migration.
quantum evolution
Proposes that the origins of major groups (families, orders, and classes) occurred as a response to drastic changes in environment or adaptive zones.
trace fossils
Fossils consisting of footprints, burrows, droppings, eggs, nests, and other types of impressions.
Points to Consider
Consider the range of tools used to study a history which no human could witness. These range from fossils – the actual remnants of living organisms – to comparisons of molecules within organisms still living today. Which tools do you consider most reliable? Does the fact that the information from one set of tools often confirms that evidence collected using a different set of tools strengthen your acceptance of the data?
Review the various patterns of macroevolution, from mass extinction to coevolution and coextinction. Which of these best support the depiction of evolution as a bush, rather than an arrow? Which support the idea that evolution builds on what already exists, so the more variety there is, the more there can be in the future?
Lesson 11.2: Early Life
Lesson Objectives
Relate the nature of science to our current understanding of the origin of life.
Describe the formation of the atoms which build the Earth and its life.
Explain the formation of the moon, and its effects on Earth’s conditions for life.
Compare and contrast Earth’s early atmosphere with today’s atmosphere.
Discuss the formation of Earth’s early atmosphere and oceans.
Indicate the age of the Earth and identify supporting evidence.
Interpret the importance of Miller and Urey’s experiment.
Relate the properties of phospholipids to the formation of the first membranes.
Compare and contrast the “genes-first” model of the origin of life to the “metabolism first” model.
Explain why some scientists believe that RNA was the basis of early life.
Evaluate the hypothesis that exogenesis explains the origin of life on Earth.
Describe the theoretical characteristics of the first cell.
Discuss the concept of a “LUCA,” or last universal common ancestor.
Indicate the origin of photosynthesis and its consequences for Earth’s life and atmosphere.
Analyze the effects of the development of atmospheric oxygen on life.
Explain the importance of the emergence of cellular respiration.
Explain the Endosymbiotic Theory of the origin of eukaryotic cells.
Evaluate the evidence for the Endosymbiotic Theory.
Identify the origins of the three major domains of life.
Analyze the evolutionary potential of the eukaryotic cell.
Discuss the pros and cons of the evolutionary “tree” as a way of depicting the evolutionary process.
Introduction
No part of the story of life holds more mystery or fascination than its ultimate origins. Cosmologists, geologists, paleontologists, and biologists have collected, compared, scrutinized, evaluated, and revised many kinds of evidence in order to see into the past. As a result, well-accepted theories now illuminate nearly 4 billion years of life’s history, 4.6 billion years of Earth’s history, and even 13.7 billion years back to the Big Bang, which began the universe as we know it. Yet until the 19th century, most people believed the Earth was just 6,000 years old. We still do not know whether life exists beyond our Earth, nor can we predict where evolution will take life on Earth in the future, and our theories leave many chapters of the story untold. As you explore the early history of life, you must remember that the nature of science is to continue to question its own conclusions, to persist in seeking new information, and to readily modify or even overturn long-accepted theories, if new evidence contradicts them. This lesson includes some of the best explanations science can currently provide for life’s origin and early evolution. A story of stardust, explosions, collisions, competition, and cooperation should not disappoint you, but it probably won’t give you all the answers you seek. If this lesson provides insight and raises more questions, you will have a firm foundation upon which to build future understanding as it unfolds. Perhaps you could join the search!
Formation of Earth: We are Made of Stardust!
We will begin our story of the origin of life by exploring the origin of the materials which build it. The materials have a beauty and diversity of their own; perhaps your study of the Periodic Table of the Elements gave you an appreciation for their variety and individuality. Earth began as the solar system began – often described as a giant rotating cloud of dust, rocks, and gas. “Dust, rocks, and gas” may not sound inspiring, but this cloud contained the 92 elements or kinds of atoms which somehow combine to form every corner – living and nonliving - of the exquisite world we inhabit. The Big Bang (9 billion years earlier!) produced the atoms of hydrogen and helium. Elements as heavy as lithium followed the Big Bang within minutes. Stars such as red giants fused hydrogen and helium nuclei to form elements from carbon (the foundation of life!) to calcium (now our bones and teeth). Supernova explosions formed and ejected heavier elements such as iron (for red blood cells). We are not just “dust.” We - and our world - are stardust!
How did this rotating cloud of stardust become our solar system? One theory suggests that a nearby supernova sent a
shock wave through the cloud, increasing its spin to form a protoplanetary disk, shown in Figure below. Most of the mass concentrated in the middle and began to heat up, but large debris and collisions resulted in concentrations of matter outside the center. Eventually, heat in the central core began nuclear fusion of hydrogen to helium, and the Sun ignited. Matter outside the Sun’s gravity separated into rings of debris, and collisions of objects within the rings formed larger objects, which eventually became the planets. Solar wind cleared much of the remaining non-planetary material from the disk.
Figure 11.19
At left is an artists conception of the protoplanetary disk, which eventually formed our solar system. At right is an X-ray image of the remnant of Keplers Supernova, SN 1604, constructed of images from NASA telescopes and observatories. Together, art and science suggest the beauty of the dust, gas, and rocks which gave birth to our earth and its life.
One of the collections of debris, approximately 150 million kilometers from the Sun, was the protoplanet Earth. Newborn, Earth was very different from the home we know today. Bombarded by debris and heated by radioactive decay and the pressure of contraction, the Earth at first was molten. Heavy elements sank to the center, and lighter ones surfaced. Heat and solar wind meant that no atmosphere and no oceans were present.
Eventually, contraction and cooling allowed formation of a crust and retention of an atmosphere. However, continued bombardment melted portions of the crust for long periods. About 4.5 billion years ago, Earth collided with another protoplanet, Theia. This “big whack” gave us our moon and tilted Earth on its current axis, leading to the seasons, which now influence so much of life’s diversity. The Big Whack may also have initiated plate tectonic activity by speeding up the Earth’s rotation. Since then, however, the moon’s tidal drag may be slowing that rotation; scientists suggest that the day/night cycle during the Hadean may have been as short as 10 hours.
As the Earth continued to cool amidst heavy bombardment, steam escaped from the crust and active volcanoes released other gases to form a primitive atmosphere, which contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, but no more than a trace of oxygen. In the absence of oxygen, no ozone layer protected Earth from the Sun’s ultraviolet rays. Between 4.2 and 3.8 billion years ago, clouds and rain formed the oceans. The oceans were olive green, and the reddish atmosphere would have been toxic to modern multicellular organisms. Yet the stage was set for life to begin.
First Organic Molecules: Hypotheses About the Origin of Life’s Chemistry
The Hadean Eon ended 3.8 billion years ago, its timeline marked by Earth’s oldest known rocks (between 3.8 and 4.2 billion years old) and oldest known minerals (formed 4.4 billion years ago). Scientists use these dates to estimate that the Earth itself is 4.6 billion years old. Evidence for life during the Hadean does not exist, although many scientists push the theoretical origin back that far. How – and when – did life arise?
Once again, we will begin with the materials of life – this time, organic molecules, made primarily of the element carbon. Most scientists agree that these organic molecules arose before cells, which we now consider essential to the definition of life. Several hypotheses and experiments suggest ways in which organic building blocks may have formed.
In 1924, Aleksandr Oparin proposed that life could have developed through gradual chemical evolution in a “primordial soup.” In 1953, Stanley Miller and Harold Urey designed a now-famous test of the hypothesis that the conditions of primitive Earth favored chemical reactions that synthesized organic molecules from inorganic precursors. Their experiment (Figure below) showed that a mixture of gases, believed to be part of the primitive Earth atmosphere, when subjected to sparks representing lightning, formed a mixture of monomers representing each of the four major groups of organic molecules. Although DNA, RNA, and polymers were absent, 13 of the 22 amino acids that make up modern protein, plus lipids, sugars, and some building blocks of DNA and RNA, were among the products of the experiment.
The “leap” from building blocks to polymers and from organic soup to individual replicating units has been more difficult to demonstrate. In the ‘50s and ‘60s, Sydney Fox showed that early Earth conditions could result in short chains of amino acids, which in turn could form enclosed spheres. Phospholipids can self-organize into membranes in a similar fashion, and cell membranes today consist primarily of a bi-layer of these lipids. Phospholipids or polypeptides could have surrounded and protected early metabolic units, forming protocells shown in Figure below, simple membrane-enclosed spaces which may have led to the later evolution of true cells.
Figure 11.20
The Miller-Urey experiment subjected a mixture of gases thought to be present in Earths primitive atmosphere to sparking, representing lightning. After one week, the nonliving system had formed 13 of the 22 amino acids which make up modern proteins, sugars, lipids, and some of the building blocks of DNA and RNA.
Figure 11.21
Phospholipids, with hydrophilic phosphate heads (P) and hydrophobic lipid tails (L) self-assemble into membranes (1) and enclosing spheres (2) which could have protected early metabolism from outside chemical disturbances.
Walter Gilbert, Carl Woese, and Alexander Rich proposed that RNA, because it can serve both catalytic and replicating functions, was the first informational molecule, and formed the “RNA World Hypothesis” for the origin of life. Sol Spiegelman created a short chain of RNA which was able to replicate itself in the presence of RNA polymerase; the segment is now known as the “Spiegelman monster.” The idea that a successful replicator molecule preceded the evolution of biochemical pathways is the “Genes-First” model.
In contrast, Günter Wächtershäuser proposed that sulfides of iron and other minerals contain energy which could have polymerized basic building blocks. He argued that extensive evolution of biochemical pathways might have preceded replicator molecules and individualization of life. His ideas formed the basis of William Martin and Michael Russell’s 2002 hypothesis that black smokers at seafloor spreading zones, shown in Figure below, could have provided conditions for extensive chemical and biochemical pathway evolution. Their reasoning suggests that lipid membranes allowing independent lives away from the smokers could have been a last step in early evolution. The fact that archaebacteria and eubacteria (and us eukaryotes!) have completely different membrane lipids but similar metabolism supports the concept of early biochemical pathway evolution. These ideas comprise the “Metabolism-First” model.
The discovery of organic molecules in space supports the exogenesis hypotheses which propose that life could have originated elsewhere – on Mars, or at some distant point in the universe. Comets and meteorites are known to contain organic molecules, and could have delivered them to Earth. Exogenesis does not really answer the question of how life originated, but provides a much wider temporal and spatial framework in which it could have happened.
Figure 11.22
Black smokers at a mid-ocean ridge hydrothermal vents could have provided conditions suitable for the evolution of early biochemical pathways and much of metabolism, even before lipid membranes formed cells. Martin and Russell propose that the last universal common ancestor may have emerged from a black smoker.
Emergence of Life: The First Cells were Prokaryotes
Although many hypotheses and some experiments and observations explore the origin of cellular life, actual events remain unknown. If earth’s life first arose on earth, rather than by exogenesis, its timing is speculative, for no fossils record that event. Admitting that many conflicting hypotheses exist, we often express our current understanding of the story this way:
Perhaps four billion years ago during the Hadean Eon, lightning and a primitive atmosphere produced an organic soup of chemicals. As the “soup” became more concentrated, molecules began to interact with one another. As molecules became more complex, some molecules helped to speed up or catalyze chemical reactions (perhaps RNA, but eventually p
rotein). Within that highly reactive soup, a molecule gained the ability to copy itself, becoming the first replicator (perhaps RNA, but eventually DNA). Copies contained errors, and errors which prevented replication caused the copies to “die out.” Copies that replicated faster survived to make more copies. Eventually, lipid membranes surrounded some of these chemicals, protecting them from reacting with other chemicals.
Although many protocell “species” probably populated the early “soup,” scientists believe that only one – a last universal common ancestor (LUCA) – emerged about 3.5 billion years ago during the Archean Eon, and later gave rise to all cellular life on earth. This prokaryote probably had a cell membrane and ribosomes, and used DNA for information storage, RNA for information transfer, and protein for catalyzing chemical reactions – like all life today. The first cells were probably heterotrophs, feeding on energy-rich chemicals concentrated in the “soup.” Alternatively, they could have been chemoautotrophs, extracting energy from inorganic molecules. Not long after prokaryotic cells emerged, they split into two major groups, Eubacteria and Archaebacteria. Both persist today, although Archaebacteria more often inhabit extreme habitats.
Inevitably, a diminishing supply of food molecules led to competition. At some point, glycolysis evolved as a pathway for transferring energy from organic molecules to ATP. This pathway persists in almost all organisms today.
Eventually, about three billion years ago, a new strategy evolved among some prokaryotes, which used sunlight to make carbohydrates from carbon dioxide and water. Photosynthesis provided a new source of food molecules for both autotrophs and the heterotrophs that “learned” to consume them. The oldest fossils, stromatolites, (Figure below) record abundant photosynthetic cyanobacteria from that time.
CK-12 Biology I - Honors Page 47