Topics Covered in this Lecture:
- Where Did All This Water Come From?
- Evolution of Early Life
- The Oxygen Holocaust
- The Appearance of Eukaryotes
- The Appearance of Multicellular Life
Suggested Reading: Microcosmos: Four Billion Years of Microbial Evolution, by Lynn Margulis and Dorion Sagan, 1986, Touchstone, Simon and Schuster
Since the dawn of time, the oceans, which now cover nearly three-quarters of our planet, have played a major role in shaping the evolution of our planet. Even today, the oceans act like a great global steam engine, trapping the energy of the sun and swirling it across the globe in great currents of water. The waters of the globe pump heat and vapor into the atmosphere, creating clouds and storms, and causing the ice caps to advance and retreat. In this lecture, we will look at the formation of the Earth's oceans and examine the evolution of the seas and their inhabitants to their current state. We will also discuss the first major pollution crisis on our planet -- the introduction of oxygen to the atmosphere by photosynthetic bacteria, sometime called the Oxygen Holocaust.
Where Did All This Water Come From?
Probably the most striking thing about the Earth is the amount of water that it contains. The famous science fiction novelist, Arthur C. Clarke once remarked (as I'm sure many other people have): "How inappropriate to call this planet Earth, when clearly it is Ocean." Surely any visitor from beyond our planet would notice that it is covered with water (if they recognized the blue appearance as water). Yet, Earth is the name we go by and so we are stuck with it for now.
There are many ways to express the amount of water on the Earth (none of which give us a real sense of the enormity of it all). The short of it is that 71% if the Earth's surface is covered by the world's oceans. The total volume of water on the Earth, including glaciers, rivers, lakes, groundwater and atmospheric water, is 1,398,898,300 cubic kilometers. For those of us who require a more practical unit of measurement, this amount is approximately equivalent to 40 thousand billion billion cans of your favorite beverage (~40 x 10^21 12 oz. cans). That's a lot of water!
Where did all this water come from? In our previous lecture we talked about the formation of the Earth from the aggregation of planetesimals to form larger bodies which we now call planets. As the Earth formed, approximately 4.5 billion years ago (BYA), nuclear heating and the energy of thousands of collisions with smaller planetesimals and meteorites caused it to become molten, at which time the core of the Earth differentiated into layers based on the density of materials within them. As the Earth's core was differentiating, gases were released. These gases bubbled to the surface where they escaped to outer space (especially the lighter gases like helium and hydrogen) or were held by the forces of gravity to form our atmosphere. This process, called outgassing, was responsible for the release of gases from the interior of the earth that formed our early atmosphere.
In its initial stages, the Earth was too hot for an atmosphere to form. Any gases were superheated and released into space. However, once the Earth cooled sufficiently, after about 1 billion years, clouds began to form in the atmosphere, and the Earth entered a new phase of development. It began to rain. And it rained and it rained and it rained.
The formation of an atmosphere also protected the Earth from smaller meteorites, which burned up before they impacted the surface. The cloudy atmosphere also reflected some of the radiation from the sun, allowing the Earth to cool further. Eventually, the surface of the Earth solidified and the crustal plates were formed. After several hundred million years, the Earth finally had oceans, and atmosphere, and continents.
In addition to outgassing, other processes released gases and were responsible for bringing water to the Earth. These include:
However, there is some evidence that these mechanisms alone could not account for all the water that we see on our planet. Currently, the most popular explanation is that after the earth cooled sufficiently, bombardment of the planets by meteorites brought water to the planet. Meteorites continue to bombard our planet every day, although we mostly see them as "shooting stars" that burn up before they hit the Earth's surface.
New clues are being provided in space. The recent Clementine mission to the moon just revealed images of what looks like ice in the dark craters of the moon's south pole. If water exists on the moon, it may provide the critical resource required to colonize the moon.
NASA scientists now believe that two of Jupiter's moons contain water. What looks like an icy ocean has been revealed in images of Ganymede taken by Galileo, an unmanned spacecraft studying the outer planets. There is also speculation that Europa houses an ocean of considerable dimensions. These new developments raise all kinds of possibilities, including life on other planets.
In 1996, astronomers discovered water in deep space. Astronomers have long speculated that water plays an important role in the formation of stars and planets but they have been unable to detect water in space because water in our atmosphere interferes with the measurement. The launch of the European Space Agency's Infrared Astronomy Satellite ISO enabled astronomers to detect water in space for the first time a carbon-rich planetary nebula (a fuzzy mass of interstellar gas and dust thought to be a place where planets form) in the constellation of Cygnus (about 2600 light years away). This instrument also detected water vapor -- appropriately -- in the giant star Hydra (which means water monster). Since then, astronomers have been able to observe water in a variety of other sources.
Astronomers consider these observations as a breakthrough in their understanding of the formation of stars and planets. Data from this satellite promises to provide keen insights into the complex chemistry that takes place in interstellar space. According to the Swedish Professor Rene Liseau of Stockholm Observatory, who analyzed these data together with his Italian colleagues, "Without exaggeration, these discoveries will be of utmost importance for our understanding of the highly complex chemistry and physics which govern the formation of stars and planets."
Question: Why is it so important to know where all the water came from on Earth?
Evolution of Early Life: Precambrian Times (4.5 BYA to 540 MYA)
The period of time from the Earth's formation to the appearance of life is known as the Hadean period, lasting from approximately 4.5 to 3.8 BYA. The appearance of life marks the beginning of the Precambrian, which lasted from 4.5 BYA to 540 MYA.
Approximately 3.5 billion years ago, Earth's atmosphere had no free oxygen and consisted mostly of hydrogen, methane, ammonia, and water vapor. The weathering and dissolution effect of the rains described above must have led to oceans that were rich in various chemical compounds. This early environment is sometimes called the primordial soup.
It was from the primordial soup -- the early ocean as I like to call it -- that the organization of chemical compounds into self-replicating molecules that led to life must have begun. Although we have not yet been able to completely duplicate the conditions and "players" (i.e. chemicals) that led to life as we know it, advances in molecular biology, non-equilibrium thermodynamics, and evolutionary symbiology, among others, are contributing to our understanding of early life and giving support to a few emerging theories.
While this is not a class in the origins of life (and all the controversy that naturally surrounds such a topic), suffice it to say that among the scientific explanations, two theories seem to be getting the most attention. The first set of experiments that gave credence to abiogenic origins of life were conducted by Miller and Urey. In 1953, two biochemists named Stanley Miller and Harold Urey boiled hydrogen, methane, ammonia, and water vapor in a flask and introduced electric sparks to simulate lightning. What they found after several days was a complex mixture of organic molecules called amino acids. Amino acids are the building blocks of all proteins, which are found in all living things. These experiments showed that pre-cursors of living matter could form from the components of the early atmosphere. These experiments were significant because they suggested one possible means for the beginning of life on Earth.
In the 1970s, another set of experiments, performed by Fox, showed a possible mechanism for the formation of "cells." Fox found that when he heated certain proteins, they spontaneously formed "microspheres", tiny protein spheres in which important cellular components might be enclosed. These experiments demonstrated that compartmentalization, thought to be critical for the assembly of certain cellular components, was possible.
Recently, as a result of studies in molecular biology, it has been suggested that nucleotides such as RNA, and not amino acids, must be the precursors to life. These arguments are based on the presence of RNA (ribonucleic acid) in just about every living organism. RNA is the principal compounds responsible for assembling proteins based on the codes provided by DNA (deoxyribonucleic acid). The key questions are whether RNA can originate from some precursors present in the early Earth environment, whether this primitive RNA can self-replicate, and whether it can be present in sufficient quantities to undergo evolution into higher, more complicated structures.
Some evidence for spontaneous assembly of nucleotides has been presented by Leslie Orgel of the Salk Institute. She was able to assembly 50-nucleotide-long molecular strands from simple carbon compounds and salt, all of which may have been present in the early Earth environment. Other researchers, such as Manfred Eigen at the Gottingen Institute in Germany, have been able to create short strands of RNA that replicate themselves. While no one has created life yet, there are promising signs that living molecules can arise from non-living molecules.
The Archaean Period (3.8 - 2.5 BYA): The First Signs of Life (3.8 BYA)
Regardless of how life arose, we know it did arise probably 3.5 -4 billion years ago, and we also have a fairly good idea that bacteria -- heterotrophic bacteria -- were the first inhabitants. Heterotrophs are organisms that get their food by eating it, as opposed to autotrophs, such as plants, which manufacture their own food. These early bacteria probably made their living by assimilating (eating) simple organic molecules, although after a while they may have begun to eat each other.
It is also quite possible that early bacteria lived under extreme conditions, such as the hot sulfur ponds that occur at Yellowstone. The recent discovery of a whole new kingdom of organisms, called the Archaebacteria, lends some support to this. These very simple and ancient bacteria have been found surviving today in hot boiling sulfur springs.
Another candidate for early life are the kinds of organisms that have been found living in deep-sea vents at the bottom of the ocean. These hot-water vents spew hydrogen sulfide and other minerals that can be used by organisms. Jack Corliss, the oceanographer at Oregon State University that discovered these organisms, thinks that early life may have formed at the boundaries between crustal plates in the warm waters of the Earth's first ocean, Panthallassa.
Remember that these first bacteria were anaerobic -- that is, they lived in the absence of oxygen. There was no oxygen in the early Earth environment, so these bacteria learned how to use sulfur compounds. As a result of their metabolism, they use and produce hydrogen sulfide, the familiar rotten egg odor. Like it or not, the early Earth smelled like rotten eggs.
The reign of the anaerobic bacteria was soon to end with the appearance of the first photosynthetic bacteria. These bacteria, ancestors of the blue-green algae (cyanobacteria), began the first global-scale pollution project, and forever changed the face of our planet. The proliferation of photosynthetic bacteria introduced oxygen to our atmosphere for the first time sometime between 3.5 and 2.5 billion years ago. These first "plants", which are popularly known as blue-green algae, formed colonies in the shallow seas which we can sea today in formations known as stromatolites.
Appearance of photosynthetic organisms (3.5 BYA)
The life history of the cyanobacteria and the formation of stromatolites is quite fascinating and we will take a few moments to review it here. Being photosynthetic, the cyanobacteria require sunlight. Thus, they would be present in shallow waters or in the lighted portions of the open sea (where we find them today). During the day, these organisms absorb sunlight and grow in mats of thin filaments. Because the cyanobacteria are composed of filaments of sticky sheaths, these mats are ideal for catching sand and debris. In addition, these organisms secrete calcium carbonate to from hardened, differentiated structures. As the organisms get buried in sand and their own skeletons, they die, and other organisms grow on top of them.. Through this process, stromatolites are formed, some exceeding thirty feet in height.
The oldest known stromatolites, dated at 3.5 billion years old, were found in western Australia in 1978. This seems to imply that life evolved quite rapidly (~500 million years), or that life's origins go even further back, perhaps to 4.5 billion years ago. At any rate, there is no doubt that 3.5 billion years ago in the continent now known as western Australia that blue-green algae were thriving. These "fossil" organisms still survive today in parts of Australia.
We also have very good evidence that within the time period from 2.5 to 3.5 billion years ago, oxygen was introduced to our atmosphere. In addition to sending anaerobic bacteria to their foul and dark haunts, oxygen had another major effect on the chemistry of our planet -- everything containing iron began to rust. Red beds of "oxidized" iron show up in rocks that are less than 2 billion years old, but don't appear in older rocks. This is clear evidence that the earth's atmosphere was changing dramatically. In fact, Lynn Margulis and Dorion Sagan have called this phenomenon the "Oxygen Holocaust."
It is likely that the first photosynthetic bacteria did not require oxygen to live, but produced oxygen as a result of photosynthetic proton pumps that provided them with reducing energy. The oxygen produced by these organisms was probably absorbed for tens of millions of years through purely chemical reactions (i.e. metal compounds, minerals in rocks, atmospheric gases). However, as the chemical reactions ran to completion, oxygen began to accumulate in bits and pieces, first in one place and then another. This localized presence of oxygen also evidenced by alternating bands of oxidized or reduced iron in rocks known as Banded Iron Formations (BIFS).
Question: How do these early fossils contribute to our understanding of living organisms today?
The Oxygen Holocaust (2.5 BYA) - The Proterozoic (2.5 BYA to 540 MYA)
About 2.5 billion years ago, all of the chemical oxygen scavenging reactions were completed, and oxygen began to accumulate in the atmosphere to its present abundance of 21%. This stabilization of the atmosphere at 21% has been maintained by our biosphere for nearly 2 billion years and it is truly one of the remarkable mysteries of our planet. If oxygen levels had gone higher, our planet would have been enveloped in fire. At lower oxygen levels, many aerobic organisms would be incapable of surviving. Thus, for some reason, and by some amazing cybernetic control system invented by microbes, the oxygen in our atmosphere is maintained at a happy medium. It should also be noted that this level of oxygen also allowed for the buildup of an ozone layer which protects living organisms from dangerous mutations by ultraviolet radiation. Sound like a science fiction story? We'll review this information in the context of Gaia at the end of the next lecture.
The accumulation of oxygen in the atmosphere catapulted the first major extinction of life in the Earth's history. Many populations of anaerobic organisms were killed off or forced to exist only in places where oxygen didn't penetrate, like deep sea muds or the bottoms of lakes. There is little doubt that vast geological, chemical, and biological changes took place as oxygen was introduced to the atmosphere. In fact, as Margulis and Sagan so rightly point out, the "industrial pollution of our present...is nothing compared to the strictly natural pollution of [these early] times."
This period of time from the beginning of life (~4 billion years ago) to a period of time 2 billion years ago is often called the Age of Bacteria. During this time, nearly all of the biochemical mechanisms that we know today had evolved. The Earth's modern atmosphere was established, and microbial life permeated the oceans, the lands, and the air, cycling gases and chemical elements across the globe. Thus was built the foundation on which all ecosystems rest -- the microbial loop.
The presence of oxygen in the atmosphere and oceans also allowed an entirely new group of organisms to evolve -- the aerobes (2.2 BYA). These organisms, of which you and I are a part, require oxygen to grow and reproduce. The ability to use oxygen has many metabolic advantages and allows exploitation of energy sources to a far greater degree than anaerobic processes. Thus, the stage was set for a whole new way of life to appear.
Among the first aerobes to appear were probably single cell bacteria, blue-green algae, and simply protists. These organisms may have been much like the simple single-celled organisms we know today. If you've ever seen the colored limestone deposits at Yellowstone, you might have some idea of how life appeared around this time.
Fossils from this period -- about 1.5 billion years old -- reveal thick-walled single-celled spheres known as acritarchs. These fossils are thought to be the cysts (equivalent to spores) of primitive algae. More highly developed and intricate acritarchs from about 1.0 billion years ago have been found in the Grand Canyon.
Appearance of Eukaryotes (1.5 BYA)
These early algae were significant for a variety of reasons, but perhaps the most important feature they exhibit is the presence of distinct organelles -- tiny "organs, really, just like our heart and lungs. The presence of organelles distinguishes eukaryotes, organisms with DNA enclosed in a membrane-bound nucleus, from prokaryotes, whose DNA and RNA are not contained within any cellular structures. Eukaryotes may be single-celled, like simple algae, or multicellular, like humans (Yes, we are eukaryotes!). Besides a nucleus, these early algae showed signs of primordial chloroplasts, called plastids. They also contained little power-generating organelles known as mitochondria.
While the evolution of eukaryotes has been debated for some time, there is now strong evidence that eukaryotes arose as a kind of cooperative effort by prokaryotes. In fact, the genetic material of the chloroplast and the mitochondria very closely resembles the genetic material of the early bacteria. Thus, you might say that a eukaryote is a group of prokaryotes living together in the same cell. This theory is known as endosymbiosis, meaning "living together within."
By joining forces, cells could achieve in harmony what they could not achieve alone. While true "multicellular" organisms had yet to arrive, eukaryotes were able to maintain a division of labor and specialization within their cells that allows them to adapt to and survive in a wide range of habitats. These organisms also became motile at this time, and developed whip-like structures called flagella that let them twirl or glide through the ocean in search of food.
Appearance of Multicellular Life (1 BYA)
The appearance of multicellular organisms represents the true expression of organismal teamwork. From this point on, life's "problems" would be solved by mutual cooperation of multicellular assemblages of cells. These "confederacies" of cells gave rise to the development of specialized structures, such as holdfasts (for hanging on to a substrate), trichocysts (stinging cells, for protection), tentacles (for gathering food), and sex organs (for reproduction).
The earliest known multicellular organisms belong to a class of plant-like animals known as volvocines (from the Latin word volvere, which means "to roll"). These simple, spherical shaped organisms literally roll through the water. They are composed of a colony of cells resembling single-celled algae, but they have two flagella and an eyespot. The simplest volvocine is a colony called Gonium socilae, made up of four cells stuck together in a jelly-like matrix. Interestingly enough (and just to "prove" that they arose from individuals), any cell in this colony-of-four is capable of swimming off and starting a colony of its own.
A more complicated volvocine is Volvox, which is made up of about half a million cells. This organism actually produces specialized cells which can break off and form their own colonies. These offspring cells represent one of the first examples of asexual reproduction. Be that as it may, some species of Volvox have evolved sexual reproduction, producing and releasing eggs and sperm to create new progeny.
Unfortunately, we don't have time to consider all the interesting evolutionary innovations that occurred in the primitive seas. However, we should note that from these first simple colonies of "algal" cells, more elaborate forms of algae evolved. One familiar example is the kelp Macrocystis, which occurs off the California coast and is one of the fastest growing plants in the world. On a good day, a kelp may grow as much as three feet. Imagine if that happened to you and me!
On the animal side, soft-bodied sponges and sea anemones are implicated as the first inhabitants of the sea. Many of the early forms of animal life were soft-bodied and left no clues as to their existence. However, a few sandstone imprints of a soft-bodied animal known as Ediacarans have been found in Australia in sediments dating back to 700 million years ago (MYA). These organisms resemble sea worms, which suggests that annelids (of which earthworms are a member) had evolved by this time. Their ancestors, what Margulis and Sagan call "little societies of cells", were far simpler than the Ediacarans and resembled Trichoplax, one of the most primitive organisms in the world discovered in the 1960s climbing the wall of an aquarium! This non-descript, multicellular version of slime is believed to be one of the earliest forms of multicellular organisms.
This marks the end of the Precambrian Geological Time Period.
Question: Why is it so important to understand the changes that occurred on our planet billions of years ago?
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