Open Questions: Evolutionary Milestones

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See also: Origins of life -- Archaea and extremophiles -- Molecular biology and genetics -- Molecular evolution -- Evolutionary theory

Introduction

Eukaryotic cells

Atmospheric oxygen

Sexual reproduction

Multicellular organisms


Recommended references: Web sites

Recommended references: Magazine/journal articles

Recommended references: Books

Introduction

We know a lot at present about the "what" of life's development on this planet, but we've figured out relatively few details about "when", "where", or "how". To address these unknowns is to guarantee ourselves a very rich supply of open questions for the foreseeable future -- and many may never allow a conclusive answer. There is so much that happened when no one was around to observe it, and even the physical evidence of it is mostly gone.

Yet there are clues everywhere. It will simply require some very good detective work to understand what they're telling us. In fact, our situation is like walking into a whole library of nothing but fiendishly intricate detective novels. In almost every volume in the library it becomes apparent fairly soon what happened. But we are kept in perpetual suspense trying to find the answers about when, where, how -- and maybe even why.

Our current ignorance spans billions of years. Up until about 3.8 billion years ago the Earth was still subject to heavy bombardment by asteroid-size objects which would have so disrupted the planet's atmosphere, land, and oceans as to make the existence of life impossible. Yet there are rocks from the Isua area of Greenland that are almost this old which exhibit (in the carbon-12 to carbon-13 ratio) chemical evidence of the existence of life. And just a little later, in rocks from about 3.5 billion years ago there are stromatolite fossils, bearing evidence that photosynthesis (in some form) had already been invented. Evidently, quite a lot happened in "only" 300 million years. But what?

We are almost as ignorant of important biological developments which took place only about 100,000 years ago -- namely the appearance on the scene of Homo sapiens. We don't even know, for sure, whether this new species made its debut only in Africa, then quickly migrated throughout Europe and Asia, or whether it evolved over the whole territory more uniformly. The evidence increasingly favors the former possibility.

Significantly, the evidence which is proving to be most useful in both of these examples -- as in most other cases as well -- is now beginning to come in a flood from our awakening understanding of molecular biology, our nascent ability to read the history written in the genetic code. It seems that the detective novels we aspire to enjoy are written in an unfamiliar language we are just learning.

Galileo was referring to the science of physics when he noted: "Philosophy is written in that great book which ever lies before our gaze -- I mean the universe -- but we cannot understand if we do not first learn the language and grasp the symbols in which it is written. The book is written in the mathematical language." Had he known what we do now of biology, he would also have mentioned another book, written in a language of biological molecules.


Eukaryotic cells

Almost everything which is now considered to be "living" is made up of cells. (Viruses excepted, which are a borderline case, because they do not exhibit metabolism.) One of the bedrock distinctions in biology is between two types of cells: prokaryotic and eukaryotic. The former are represented by bacteria. The latter are represented by... just about everything else -- protists (e. g. amoebae), fungi, plants, animals.

The simplest description of the distinction between prokaryotic and eukaryotic cells is that the latter have a well-defined cell nucleus, as well as various other organelles, such as mitochondria and chloroplasts, while the former lacks all of these. Here's a more extensive list of the differences.

Prokaryotic cells Eukaryotic cells
DNA is contained in cell cytoplasm. DNA is contained in a membrane-enclosed nucleus, and is bound up with proteins in the form of chromosomes.
Has no internal membrane-enclosed organelles. Has internal organelles, such as mitochondria and chloroplasts.
Reproduces by mitosis (cell division). Capable of another division process (meiosis) in addition to mitosis.
Rigid cell wall. Flexible cell wall (membrane).
No internal cytoskeleton. Has an internal protein cytoskeleton consisting of microtubles and filaments.
Cells are small -- about 1 to 3 microns (10-6 meters). Cell volume may be 104 times as large.
Limited amount of DNA (about 5 million base pairs, several thousand genes). Much more DNA (up to several billion base pairs, tens of thousands of genes).
Genes are usually continuous segments of DNA. Multiple genes may be adjacent and transcribed simultaneously. Genes are usually discontinuous segements (interrupted by "introns").
Relatively simple process of DNA transcription and gene regulation. More complex process of DNA transcription and gene regulation, using different enzymes.

The last point on this list is one of the most significant. DNA transcription refers to the process of copying the genetic information contained in the DNA into another molecule called transfer-RNA (tRNA). Essentially, it is a straightforward copying of nucleotides (with substitution of uracil for thymine). In both prokaryotic and eukaryotic cells it is performed by an enzyme called RNA polymerase (which is slightly different between the two cases). In prokaryotic cells, genes within the DNA are marked by a region just a little "upstream" known as a "promoter". The RNA polymerase attaches to the promoter region with the help of a "sigma peptide" in order to begin transcription. Eukaryotic cells have analogues of all these features, but a lot of other machinery as well, in the form of such things as "transcription factors", "activators", "repressors", and "enhancer" and "silencer" regions of DNA.

The main effect of these differences is that prokaryotic cells can regulate gene expression (i. e., translation into proteins) in order to adjust to the environment. But eukaryotic cells can regulate gene expression much more flexibly -- with the main advantage of being able to delelop into a wide variety of cell types (about 200 in the human body) from the same DNA. This is obviously necessary in order to have complex multicellular organisms. But of course, this end result is not "why" the differnce came about. The change took place -- for reasons which are mostly unknown and at this point mysterious -- and undoubtedly it was very gradual. It just happened that the ever increasing flexibility of gene expression (which certainly had its value as it occurred) ultimately resulted in the ability for complex organisms to emerge.

At this point, we can mostly only speculate as to what sequence of events led from prokaryotic to eukaryotic cells and when the important steps occurred. Physical evidence for this evolution is very sparse. At best, we have impressions or casts of primitive organisms. But because this was billions of years before organisms had hard parts that could actually survive in rock, that's it. We have little to go on directly regarding internal structure. We certainly have no DNA samples, and probably never will.

This evolution must have occurred in a series of steps. The first was probably not the addition of something, but the loss of something. Bacteria have a rigid cell wall, which provides several advantages, such as protection of the cell from the environment, dimensional stability, and a place for attachment of the chromosome. Losing this rigid wall would have presented problems, but also a different set of advantages. For example, a thin, flexible wall could allow the cell to engulf other cells or cellular material as food -- a process known as phagocytosis.


Sexual reproduction


Multicellular organisms



Recommended references: Web sites

Site indexes


Sites with general resources


Photosynthesis and cyanobacteria

Photosynthesis
Article from Wikipedia. See also Cyanobacteria.
UCL Photosynthesis Research Group
Home page of a research group on photosynthesis at University College London. Describes group activities and contains external links.
Fossil dates Earth's first breath
August 1999 news article about fossil evidence of cyanobacteria from 2.5 billion years ago.


Endosymbiosis

Endosymbiotic hypothesis
Article from Wikipedia. See also Symbiogenesis.
The Nuclear Family
November 2001 Scientific American In Focus article, subtitled "Can the same process that introduced the mitochondrion and chloroplast explain where the nucleus came from?"
Photosynthesis's Purple Roots
September 2000 Scientific American news article about the role of purple bacteria in the origins of photosynthesis.


Eukaryotes

Eukaryote
Article from Wikipedia. See also Prokaryote, Mitochondrion.
When did eukaryotic cells first evolve?
Scientific American page with answers and external links from several experts.
The Eukaryote Cell: A Virtual Tour
Provides a description of the main features of eukaryotic cells. Part of a cell biology course at the University of Nebraska. Good external links.
Prokaryotes vs. Eukaryotes
Tabular presentation of some of the main differences between prokaryotic and eukaryotic cells (in PDF format). Part of MIT OpenCourseWare: 7.28 Molecular Biology, Spring 2001.


Sexual reproduction

The Origin of Sex: Cosmic Solution to Ancient Mystery
July 2001 article from Space.com. Discusses recent research that "has used digital organisms to simulate life before sex and yielded a possible mechanism for instigating Earth's first courtship."


Multicellular organisms


Miscellaneous

Life on Earth suddenly a billion years older
October 1999 news article about discovery of evidence for land-based oxygen-breathing life 2.3 billion years ago.


Recommended references: Magazine/journal articles

From Simple To Complex
Jef Akst
The Scientist, January 2011
The switch from single-celled organisms to ones made up of many cells has evolved independently more than two dozen times. What can this transition teach us about the origin of complex organisms such as animals and plants?
When Methane Made Climate
James F. Kasting
Scientific American, July 2004
Questioning the Oldest Signs of Life
Sarah Simpson
Scientific American, April 2003
Pass the Genes, Please
John Travis
Science News, July 22, 2000, pp. 60-61
Lateral gene transfer in early bacteria makes it much more difficult to reconstruct the evolutionary history of the earliest prokaryotes.
The Birth of Complex Cells
Christian de Duve
Scientific American, April 1996, pp. 50-57
Biologists have traditionally classified cells as prokaryotic (bacteria), which have a simple structure, and eukaryotic (most everything else), which have a more complex internal structure. Eukaryotic cells seem to have evolved out of the symbiosis of various types of simpler cells.
Living Together
John Rennie
Scientific American, January 1992, pp. 122-133
Parasites probably play a larger role in the process of evolution than is generally suspected. Even sexual reproduction may have evolved to allow organisms to cope with parasites.


Recommended references: Books

Mark Ridley -- The Cooperative Gene: How Mendel's Demon Explains the Evolution of Complex Beings
The Free Press, 2001
Ridley, a highly resptected expert on evolution, considers the problem of how multicellurlar life could have evolved. It is a problem for two reasons: because genes have a predisposition to "selfisheness" and because the sheer number of genes in a multicellular organism brings vulnerability to harmful mutations. But Ridley argues that there is also a tendency he calls "Maxwell's Demon" to reward cooperative genes in multicellular organisms.
J. William Schopf -- Cradle of Life: The Discovery of Earth's Earliest Fossils
Princeton University Press, 1999
Schopf provides a fine presentation of the development of life from its murky origins through the appearance of eykaryotic cells. The treatment focuses on the history of major discoveries and is aimed at general readers. Schopf himself is responsible for the discovery of the oldest known fossils of living organisms, about 3.5 billion years in age.

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Copyright © 2002 by Charles Daney, All Rights Reserved