The question of how life began has been with us since the beginnings or recorded history. It is now accepted that there was a time, however brief or long, when the earth was a lifeless (prebiotic) planet. Life’s origins on earth date to some 3.7-4.1 billion years ago under conditions that favored the formation of the first cell, the first entity with all of the properties of life. But couldn’t those same conditions have spawned multiple cells independently, each with all of the properties of life? If so, from which of these did life, as we know it today, descend? Whether there were one or more different “first cells”, evolution (a property of life) only began with those cells.
115 Properties of Life
The fact that there is no evidence of cells of independent origin may reflect that they never existed. Alternatively, the cell we call our ancestor was evolutionarily successful at the expense of other life forms, which thus became extinct. In any event, whatever this successful ancestor may have looked like, its descendants would have evolved quite different appearances, chemistries and physiologies. These descendant cells would have found different genetic and biochemical solutions to achieving and maintaining life’s properties. One of these descendants evolved the solutions we see in force in all cells and organisms alive today, including a common (universal) genetic code to store life’s information, as well as a common mechanism for retrieving the encoded information. Francis Crick called is commonality the “Central Dogma” of biology. That ancestral cell is called our Last Universal Common Ancestor, or LUCA.
116 The Universal Genetic Code 117 Origins of Life 118 Life Origins vs Evolution
Elsewhere we consider in more detail how we think about the origins of life. For now, our focus is on evolution, the property of life that is the basis of speciation and life’s diversity.
Natural selection was Charles Darwin’s theory for how evolution led to the structural diversity of species. New species arise when beneficial traits are naturally selected from genetically different individuals in a population, with the concomitant culling of less fit individuals from populations over time. If natural selection acts on individuals, evolution results from the persistence and spread of selected, heritable changes through successive generations in a population. Evolution is reflected as an increase in diversity and complexity at all levels of biological organization, from species to individual organisms to molecules. For an easy read about the evolution of eyes (whose very existence according to creationists could only have formed by intelligent design by a creator), see the article in National Geographic by E. Yong (Feb., 2016, with beautiful photography by D. Littschwager).
Repeated speciation occurs with the continual divergence of life forms from an ancestral cell through natural selection and evolution. Our shared cellular structures, nucleic acid, protein and metabolic chemistries (the ‘unity’ of life) supports our common ancestry with all life. These shared features date back to our LUCA! Most living things even share some early behaviors. Take our biological clock, an adaptation to our planet’s 24 hour daily cycles of light and dark that have been around since the origins of life; all organisms studied so far seem to have one!. The discovery of the genetic and molecular underpinnings of circadian rhythms (those daily cycles) earned Jeffrey C. Hall, Michael Rosbash and Michael W. Young the 2017 Nobel Prize in Medicine or Physiology (click Molecular Studies of Circadian Rhythms wins Nobel Prize to learn more)!
The molecular relationships common to all living things largely confirm what we have learned from the species represented in the fossil record. Morphological, biochemical and genetic traits that are shared across species are defined as homologous, and can be used to reconstruct evolutionary histories. The biodiversity that scientists (in particular, environmentalists) try to protect is the result of millions of years of speciation and extinction. Biodiversity needs protection from the unwanted acceleration of evolution arising from human activity, including blatant extinctions (think passenger pigeon), and near extinctions (think American bison by the late 1800s). Think also of the consequences the introduction of invasive aquatic and terrestrial species and the effects of climate change.
Let’s look at the biochemical and genetic unity among livings things. We’ve already considered what happens when cells get larger in evolution when we tried to explain how larger cells divided their labors among smaller intracellular structures and organelles. When eukaryotic cells evolved into multicellular organisms, it became necessary for the different cells to communicate with each other and to respond to environmental cues.
Some cells evolved mechanisms to “talk” directly to adjacent cells and others evolved to transmit electrical (neural) signals to other cells and tissues. Still other cells produced hormones to communicate with cells to which they had no physical attachment. As species diversified to live in very different habitats, they also evolved very different nutritional requirements, along with more extensive and elaborate biochemical pathways to digest their nutrients and capture their chemical energy. Nevertheless, despite billions of years of obvious evolution and astonishing diversification, the underlying genetics and biochemistry of living things on this planet is remarkably unchanged. Early in the 20th century, Albert Kluyver first recognized that cells and organisms vary in form appearance in spite of the essential biochemical unity of all organisms (see Albert Kluyver in Wikipedia). This unity amidst the diversity of life is a paradox of life that we will probe further in this course.
A. Genetic Variation, the Basis of Natural Selection
DNA contains the genetic instructions for the structure and function of cells and organisms. When and where a cell or organism’s genetic instructions are used (i.e., to make RNA and proteins) are regulated. Genetic variation results from random mutations. Genetic diversity arising from mutations is in turn, the basis of natural selection during evolution.
B. The Genome: An Organism’s Complete Genetic Instructions
We’ve seen that every cell of an organism carries the DNA including gene sequences and other kinds of DNA. The genome of an organism is the entirety of its genetic material (DNA, or for some viruses, RNA). The genome of a common experimental strain of E. coli was sequenced by 1997 (Blattner FR et al. 1997The complete genome sequence of Escherichia coli K-12. Science 277:1452-1474). Sequencing of the human genome was completed by 2001, well ahead of the predicted schedule (Venter JC 2001The sequence of the human genome. Science 291:1304-1351). As we have seen in the re-classification of life from five kingdoms into three domains, nucleic acid sequence comparisons can tell us a great deal about evolution. We now know that evolution depends not only on gene sequences, but also, on a much grander scale, on the structure of genomes. Genome sequencing has confirmed not only genetic variation between species, but also considerable variation between individuals of the same species. Genetic variation within species is in fact the raw material of evolution. It is clear from genomic studies that genomes have been shaped and modeled (or remodeled) in evolution. We’ll consider genome remodeling in more detail elsewhere.
C. Genomic ‘Fossils’ Can Confirm Evolutionary relationships.
It had been known for some time that gene and protein sequencing could reveal evolutionary relationships and even familial relationships. Read about an early demonstration of such relationships based on amino acid sequence comparisons across evolutionary time in Zuckerkandl E and Pauling L. (1965) Molecules as documents of evolutionary theory. J. Theor. Biol. 8:357-366. It is now possible to extract DNA from fossil bones and teeth, allowing comparisons of extant and extinct species. DNA has been extracted from the fossil remains of humans, other hominids, and many animals. DNA sequencing reveals our relationship to each other, to our hominid ancestors and to animals from bugs to frogs to mice to chimps to Neanderthals to… Unfortunately, DNA from organisms much older than 10,000 years is typically so damaged or simply absent, that relationship building beyond that time is impossible. Now in a clever twist, using what we know from gene sequences of species alive today, investigators recently ‘constructed’ a genetic phylogeny suggesting the sequences of genes of some of our long-gone progenitors, including bacteria (click here to learn more: Deciphering Genomic Fossils). The comparison of these ‘reconstructed’ ancestral DNA sequences suggests when photosynthetic organisms diversified and when our oxygenic planet became a reality.