Who is related to whom? 


The interrelationship of the living forms of organisms on earth is both fascinating and complex.    The above diagram depicts the evolutionary continuum from simple, single celled organisms of some 3.5 billion years ago to the complex plants and animals we find on the earth today.

The diagram is based on data from the Tree of Life Project at http://tolweb.org/tree/phylogeny.html as well as the textbook “Invertebrate Zoology” by Ruppert, Fox and Barnes, 2004.

Each name shown on the chart is that of a major group of related organisms called a Phylum.  The colored names are ones that are discussed on this web site.

The relationships depicted on this chart are based on anatomical, reproductive and molecular biological studies.  However there is still disagreement between scientists on some of the relationships.

The major advances in the evolution of complex life forms begins with the enclosure of various biologically active molecules and more complex structures, such as DNA and Ribosomes, within a cell membrane to form a prokaryotic cell.  Bacteria and blue-green algae are examples of a prokaryotic cell.  As this simple cell organization faced the variety of different environments in the nearshore and deeper waters of the primal ocean, communal interaction between prokaryotes gave rise to a more complex cell, called a eukaryote, which subsequently evolved into a wide variety of multicellular organisms.

Some of those multi celled organisms evolved the ability to capture sunlight and are now known as plants.  Others were consumers of organic material and became animals.  Originally the animals were symmetrical in design, such as the jellies and ctenophores or lacked symmetry such as the sponges.  However early on some forms developed bilateral symmetry (or a right and left side) which led to all of the more complex animals we see today.  In addition a major difference between the bilaterally symmetrical animals, based on early life history development, seperates one group leading to sea stars, acorn worms, tunicates and vertebrates from another group that includes worms, clams and crabs.  Finally the worms, which have chitinous bristles (chaeta), are distinct from insects and crustaceans which molt their external skeletons.