paleontology 1: The most fundamental evolutionary transition of them all. (v1.0)

The key references for all the set of Paleontology essays are Dr Anthony Martin of Emory University and Dr John Hawks of University of Wisconsin-Madison. If you are more visual, you may like the series on paleontology in Netflix called “Life on our planet”. 

Paleontology is the study of ancient life – all the way from the simplest lifeform like a single celled life to humans. Human evolution from apes is a specialty of paleontology called paleoanthropology. I have already covered the evolution of humans from their ancestors. 

The story of life on earth stretches back 3.7 to 4 billion years. Earth is 4.5 billion years old. There is a mountain of evidence backing up what paleontology says, derived from varied disciplines like fossil studies, biology, genetics, geochemistry, molecular biology, geology, etc. I will not dwell into this evidence or the scientist’s making those discoveries, or the natural selection factors that pushed it, but tell a simple story of the major/key evolution transitions of life from its simplest form to the ancestors of humans (homo genus) and other lifeforms present today. This is the first of multiple essays on this subject.

What did the planet look like when life first appeared? The planet looks vastly different to the one we know today. Of course, with no life, there are no trees, plants, or animals. There is also no oxygen or ozone layer, leaving the planet’s surface exposed to the sun’s intense UV rays and making it blisteringly hot. Despite the hot temperatures, there are vast oceans covering most of the surface of earth. Frequent volcanic eruptions spew gases into the atmosphere, including carbon dioxide, nitrogen, hydrogen, and water vapor. There were frequent meteor showers. Not a wonderful place to be, by any standards.

I will start at the beginning when life first appeared – four billion years ago. The Precambrian geological era stretches from over four billion years ago to about 541 million years ago. No one really knows how DNA and RNA (some think RNA formed before DNA), which are the building blocks of life on earth, first formed from the soup of chemicals present in early earth. Did RNA come before DNA, or did they appear together? Did it happen spontaneously? Did it come from outer space – say a meteorite? Was it seeded by aliens or some divine intelligence? I will not really dwell on that.

The first single celled organisms are Prokaryotes that appeared about 3.5 to 3.7 billion years ago. Prokaryotes are organisms that lack a nucleus and other membrane bound organelles (organized or specialized structures in a cell). Examples are bacteria, Archaea (some specialized life forms that survive in extreme conditions like very salty, very hot, very cold, very acidic, very alkaline, produce methane, etc.) and cyanobacteria (blue-green algae). It took over one billion years for the transition to nucleated cells to fully establish. The emergence of nucleated life is a major evolutionary transition. The first rudimentary single or multicellular nucleated life, called Eukaryotes, appeared around 2.2 billion years ago, which is an organism consisting of a cell or cells in which the genetic material is DNA in the form of chromosomes contained within a distinct nucleus as well as membrane-bound organelles (specialized structures within a living cell). Eukaryotes include all living organisms other than Prokaryotes. Examples of simple single celled Eukaryotes are Amoeba and Paramecium (what you studied in school!!).

Multicellular Eukaryotes that cannot manufacture their own food are called Metazoans (commonly called animals). Animals are also called heterotrophs. They first appeared about six hundred million years ago. The transition to nucleated multicellular animal life is the most important and fundamental evolutionary transition because all more complex animals that came after that hinged on this transition. Example of early Metazoans are Ediacaran Faunas (soft bodied). The Ediacaran Faunas are followed by an extraordinary explosion of Metazoan evolutionary radiation in the fossil record in the Cambrian period. Plants which are also evolved multicellular Eukaryotes can manufacture their own food from sunlight and carbon dioxide. Plants are also called autotrophs (Plants technically are the autotrophs on land only. An aquatic example is Phytoplankton or Algae). Autotrophs emerged before heterotrophs (autotroph fossils appeared at least two billion years ago). Fungus is another category of Eukaryotes (and emerged at least 1.2 to 1.5 billion years ago). Another category are Protists, which are both unicellular or multicellular, diverse, and mostly aquatic organisms that are not animals, plants, or fungi.

 This essay has covered the two major evolutionary transitions between four billion years and about 600 million years. A vast stretch of time indeed!!