Origins of Life


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.

Recall the Characteristics of Living Matter that we studied earlier. What are some of the other properties of life besides replication that must be met for organic molecules to be considered living?

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.

Question: Why is it so important to understand the changes that occurred on our planet billions of years ago?

Recommended Reading:

Please read the brief and nicely illustrated goals of NASA's Astrobiology Program. They will give you keen insights as to theories of the origins of life on our planet.

NASA Astrobiology Program Goals
http://astrobiology.arc.nasa.gov/roadmap/goals/index.html

Please also take a look at these pages on Endosymbiosis.

Endosymbiosis: The Evolution of Life
http://bluegrass.rs.itd.umich.edu/~bhacker/endo.html

Useful Links:

NASA Astrobiology Program
http://astrobiology.arc.nasa.gov/home.html

Endosymbiosis and the Origins of Eukaryotes
http://people.ne.mediaone.net/jkimball/BiologyPages/E/Endosymbiosis.html