From my career as a technical writer in biotechnology—first with a pharmaceutical company making biological products from recombinant DNA, then with an instrument company making tools for genetic analysis—I have remained fascinated with life’s processes and how they developed.1
The first life on Earth was microscopic: bacteria and single-celled organisms. They appeared about 3.6 billion years ago, just shortly after the solar system and the planets formed, and just about as soon as the Earth’s surface and atmosphere cooled enough to allow life processes to form without being cooked. The planet’s original atmosphere had no free oxygen—that came later, with photosynthesis in plants. The earliest bacteria loved hot environments and made their living by converting sulfur and carbon into energy-bearing chemicals.
With no cell nucleus and no nuclear processes, the DNA of microbes—called the “prokaryotes”—is present throughout the cell body. All of their DNA is continuously being transcribed to RNA and translated to proteins. That is, the cell makes all of its component parts all the time, and so it endlessly expands. However, the cell wall or membrane is not infinitely elastic. It would simply split open and spill its contents if it did not find a way to copy and separate those strands of DNA, attach the new copies to different parts of the cell membrane, and then divide in an orderly fashion to make two daughter cells in a process called “binary fission.” Microbes grow and divide and grow again by sheer necessity. The growth of microbes is uncontrolled—except by the availability of water, light, and nutrients—unless adverse conditions intervene.2
This was the nature of life on Earth for about 1.5 billion years. Then the first “eukaryotes” appeared. Single cells had started banding together into primitive organisms—the first being a coiled, multi-cellular algae found in iron formations in Michigan dating back 2.1 billion years. Eventually, the intracellular DNA was collected in a single structure within the cell body, the nucleus, which enabled selective gene promotion and cellular specialization.3 And other complex structures call organelles were suddenly found inside the cell body—likely acquired as separate, single cells themselves which were absorbed by the parent. Probably the most familiar of these is the mitochondrion, which processes food energy into the cell’s basic energy molecule, adenosine triphosphate. The mitochondrion has its own DNA, separate from the cell’s nuclear material.4
Life on this level proceeded for another billion years or so. And then, about 500 million year ago the natural world exploded: multi-celled organisms diversified rapidly, and the major animal phyla all appeared within about 20 million years. And all of these animals are related.
The clue to this relationship can be found in the similar molecular structures, drawing on identical protein-coding genes, shared by widely divergent genuses. A specific example can be found in the work of Eric Davidson, a researcher at the California Institute of Technology. He and his team have studied the development of sea urchin embryos on an almost minute-by-minute basis after fertilization. He found that once the blastula—the globe of cells resulting from divisions in the fertilized egg—was formed, cells in different parts of the globe began differentiating.5 Some became the calcium-secreting cells that would form the creature’s skeleton, others became the precursors to the gut and the outer skin. All of this was based on the cell’s original position in one or another quadrant of the blastula.
Further, Davidson studied the genetic activity inside those cells and discovered that one patch of DNA always transcribed a small RNA strand that never left the nucleus but instead found and annealed to another part of the DNA. That triggered transcription of a second RNA strand, which then triggered a third RNA strand, and on and on. Depending on position in the blastula and timing, these RNA strands branched along different networks that defined the particular cell type and its activity. Davidson then studied embryo development in other animals that had not shared an ancestor with the sea urchin in about 300 million years and found the same early development networks. That means humans probably share the early stages of this development pattern, too, which then diverges depending on the DNA in our genome and the genes that are finally triggered to create cellular proteins.6
In the dry language of genetic analysts, the roots of life—its development from simple to complex chemistries, and from simple to complex structures—are “highly conserved.”
But the point of this meditation is actually the timeline itself. Life arose almost immediately on this planet, 3.6 billion years ago.7 It took about half that time, 1.5 billion years, to progress from simple, single-celled prokaryotes to the more complex, multi-celled eukaryotes. And about a quarter of that time, 1 billion years, to develop complex and highly diversified physical forms. Finally, only in the last fraction of life’s time on Earth, about 65 million years during the age of mammals, after the Chicxulub meteor strike wiped out the dinosaurs and let our primitive shrew-like ancestors evolve, did truly complex, non-instinctual, self-perceiving mental structures—human-scale intelligence—develop. The simplest advances took the longest time; the more complex advances took less and less time at each stage. And the part of the story we have the hardest time understanding—development of our own brains—took almost no time at all in the geologic record.
You would think life would progress the other way around: baby steps coming fast at first, and then the development of an ever richer complexity taking longer and longer. The fact that life arose in shorter and shorter timeframes like this tells me the driver was not some implicit plan or guiding intelligence. Complexity has a power of its own. Richness and diversity beget more and more richness and diversity. It reminds me of the sweep of science and technology: since the first scientific discoveries during the Enlightenment, our understanding and application has been moving faster and faster.8
Isn’t this is a wonderful time to be alive?
1. See for example, Evolution and Intelligent Design from February 24, 2013, and DNA is Everywhere from September 9, 2010.
2. For example, if conditions suddenly become too extreme—too little food, too cold, too dry—the single cell can pack its DNA into a tight, tough little package called a spore and release it to await better conditions. The cell dies but the DNA travels onward. The decision to form a spore is automatic, driven by transcription factors in the DNA that respond to certain chemical cues.
3. Among the various specializations was the differentiation of body parts and individuals into male and female. And so sexual reproduction was born.
4. You don’t get your mitochondria from both parents. Instead, you inherit it strictly from the egg donated by your mother; all mitochondria come from the female line. Also, you have many more copies of the mitochondria with its specific DNA scattered within a single cell than you have copies of the cell’s nuclear DNA. This is why isolating and studying mitochondrial DNA is usually easier in ancient or damaged tissues than studying the nuclear DNA.
5. For illustrations of this process, see Davidson College’s “Introduction to Sea Urchin Development” and Stanford University’s “Sea Urchin Embryology.”
6. The preceding paragraphs are copied from my earlier blog “Gene Deserts” and the Future of Medicine from December 5, 2010—another meditation on the nature of DNA.
7. And yet the complex chemistry that enables self-replicating life to function in the first place—the interplay of ribose sugar rings and phosphate bonds in DNA and RNA, and their intricate mediation between amino acids and proteins—seems to have been present almost from the beginning. As noted in Communicating with Aliens from July 28, 2013, neither the fossil record nor the far-flung examples of life on this planet suggest any evolution of this DNA/RNA/protein coding system from a more primitive variant. That suggests it either arose awfully fast and quickly outcompeted any other basis for life—or it came here in its fully developed form from somewhere else, either seeded on this planet or left inadvertently as a microbe on some astronaut’s glove.
8. See The Dollar Value of Technical Advances from May 5, 2013.