9.II: Oxyen and Eukaryotic Cells

diatoms

The proterozoic eon was the age of protists: Single-celled eukaryotes. They were made possible by the acquisition of oxygen-fueled mitochondria. Protists were the dominant life form for a billion years, longer than animals have existed. In that time, even one class of protists called diatoms was able to evolve the diversity seen here.

In Chapter 10, bacteria and archaea evolved many different metabolic pathways, or means of getting sustenance from the environment.  The earliest cells were anaerobic; they used chemical pathways such as glycolysis and fermentation that do not require oxygen.  Originally, the air and sea were devoid of oxygen.  In fact, that was an important key condition for abiogenesis and the very first signs of life.  Oxygen is a highly destructive gas; after all, it is the fuel for rust, fire, and explosions!  Without proper protection from cell membranes, life’s earliest experimental molecules would have been destroyed by oxygen. 1

Around 2.5 or 3 billion years ago, two very special bacteria evolved that would literally change the atmosphere forever.  A cell called cyanobacteria developed photosynthesis.  Photosynthetic cells use solar energy to produce their own ATP.  They release oxygen gas as a waste product.  An ocean full of cyanobacteria pumped out a great deal of oxygen.  For a few hundred million years, this oxygen got absorbed by iron and carbon, getting harmlessly locked away in the form of rust or carbon dioxide.  The ocean floors got covered with a reddish rust sediment. 2 When those sinks became saturated, the oxygen started to overflow.  It aerated the ocean and bubbled up into the atmosphere, setting off the Great Oxygenation Event (GOE).    What an episode of global pollution!  Here was a gas that was deadly poisonous to most forms of life at the time, becoming increasingly concentrated throughout the entire world.  It must have certainly caused a major mass extinction, though there is no way to find direct evidence of this anymore.  The anaerobic survivors have since been relegated to strange anoxic environments like deep sea vents, swamps, sewage, and the guts of animals. 3

Fortunately, alpha-proteobacteria evolved gradually around the same time.  These cells were aerobic; they had the ability to use oxygen to make their ATP.  The byproduct of aerobic respiration is carbon dioxide, which is required by photosynthesis.  This established a mutually beneficial cycle between the cyanobacteria and α-proteobacteria of the world.  Aerobic respiration produces much more energy than anaerobic respiration, so α-proteobacteria became highly successful as the world became more oxygenated.

The Great Oxygenation Event had another positive outcome.  Some atmospheric oxygen became ozone.  Ozone floats to the top of the atmosphere and creates a protective layer against the sun’s most harmful ultra-violet radiation.  Prior to the GOE, the only place on Earth safe from killer sunburn was beneath the ocean waves.  The ozone layer set the stage for life to crawl out onto land.

The more the oceans become oxygenated, the more difficult life became for the old-fashioned anaerobic cells.  Eventually, they had no choice but to partner up.  In a process called endosymbiosis, some of the largest anaerobic cells (probably archaea) literally swallowed α-proteobacteria whole and allowed them to live inside! 4 The aerobic passenger cells absorbed the host cells’ “toxic” oxygen and provided a generous supply of ATP.  In exchange, the passenger cells enjoyed a hospitable environment inside their hosts.

With their amazing new power supplies, host cells gradually grew larger and more complex.  They were the first eukaryotic cells, which make up all plants and animals today.  A key development of these hybrid cells was growth of the membrane.  The membrane formed folds that worked their way into the interior of the cell.  Membranous folds greatly increased the cell’s available surface area for embedding proteins, which perform an endless variety of life functions. 5 Other portions of the membrane formed bubbles, cells within the cell.  All of these various structures eventually came to assume specialized roles as the cell’s organelles, which you probably remember memorizing in high-school biology.  For example, internal folds of membrane called ER are used to make hormones, embed ribosomes, and create channels and compartments within the cell.  Bubbles of membrane called vesicles can contain molecules and safely transport them around.

Eventually, the network of membranes formed a full inner enclosure, the nucleus, surrounding the DNA.  DNA was now doubly-protected from the outside world.  In eukaryotes, DNA took the form of long chains called chromosomes.  As for the α-proteobacteria, the hitchhiker who provided all the energy for this cell growth in the first place, it evolved into what we now call the mitochondria of the eukaryotic cell.  It ended up transferring much of its own DNA to the host cell’s nucleus. 6 It thus lost the ability to live alone, becoming a dependent organelle itself.  On the other hand, the cell cannot survive without the mitochondria either.  The eukaryotic cell became a single new unit of life.

By about two billion years ago, our ancestors were such single-celled eukaryotes called protists, a semi-accurate term meaning “the first.”  They probably started out looking much like the blobby amoebas of today.  Over the next billion years, protists developed immense diversity.  Our most recent protist ancestors came to assume the shape of opisthokonts, cells with a single tail in the rear for propulsion.  Sperm cells, arguably the oldest cells in the body, still retain this form.

In a similar endosymbiotic process several hundred million years later, cyanobacteria took up residence in some protists.  These hybrid cells assumed photosynthetic capability.  The cyanobacteria gradually gave up much of their DNA and become organelles called chloroplasts.  Photosynthetic protists, algae, were the precursors to plants.  A visitor to Earth a billion years ago might have seen the familiar green film of algae on water and wet rocks – the first visible sign of life on the planet.

It might seem so far that the pace of archaic life was incredibly slow.  We have already spanned three billion years of life, and there are still no signs of plants or animals!  The time scale is actually a testimony to nature’s job at hand.  It is much easier to combine cells together and make an animal than it is to make the cells themselves.  (Just as it is much easier to make a Lego house, train, or robot than to manufacture Legos.)  Animal diversity really amounts to just arranging cells in different ways.  The cells themselves are diverse at the deepest level, in their very chemistry.  Some cells thrive in conditions that would kill other cells instantly.  Some breathe oxygen in and others breathe it out.  Some cells have walls, some have tails.  Some cells eat others, and some are solar powered.  This kind of complexity and diversity took eons to evolve.  Cells were already three billion years old when animals started to swim the seas.  Since then, all of animal life has spanned only 800 million years.  Before animals were possible, individual cells had to find a wide range of solutions to life’s problems.  Single-celled microbes evolved the basic means to metabolize, to explore the environment, and, as we’ll see now, to reproduce.

Back to Section 9.I:  Geological Time Periods

Continue to Section 9.III:  Sexual Reproduction

  1. Diatom image:  By Wipeter (Own work), GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or FAL, https://commons.wikimedia.org/wiki/File%3ADiatom2.jpg (accessed 6/24/15)
  2.  BBC, Early Life: Oxygen enters the atmosphere, http://www.bbc.co.uk/science/earth/earth_timeline/first_life (accessed 5/14/15)
  3. Net Industries, “Anaerobic – anaerobic organisms”, http://science.jrank.org/pages/320/Anaerobic-Anaerobic-organisms.html (accessed 6/13/15)
  4. Koonin, Eugene, “The origin and early evolution of eukaryotes in the light of phylogenomics,” Genome Biology 2010, 11:209, http://genomebiology.com/2010/11/5/209, ; weighed and supported by Gray, Michael, “Mitochondrial Evolution,” Cold Spring Harbor Perspectives in Biology, 2012, http://cshperspectives.net/content/4/9/a011403.full#xref-ref-82-1, both accessed 3/11/15.
  5. McDarby, Michael, “Structures in Eukaryote Cells,” PEOI, 2012, http://www.peoi.org/Courses/Coursessp/biology/ch/ch18a.html, accessed 3/11/15.
  6. Timmis et al, “Endosymbiotic gene transfer: organelle genomes forge eukaryotic chromosomes,” Nature Reviews / Genetics, Vol. 5, Feb. 2004, p. 123, http://www.nature.com/scitable/content/endosymbiotic-gene-transfer-organelle-genomes-forge-eukaryotic-13997492  , accessed 3/11/15.  This is a very informative and well-organized paper.  It is illustrated, and its bibliography highlights the historical significance of key papers in the field.
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