See also: Origins of life -- Archaea and extremophiles -- Molecular biology and genetics -- Molecular evolution -- Evolutionary theory
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.
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.
Copyright © 2002 by Charles Daney, All Rights Reserved