THE GROUPS AND ANCESTRY OF ORGANISMS 



105 



3. Enterocoela — coelom originates as pouches 

 from the embryonic gut 



a. With secondary, usually five-rayed, radial 



symmetry 

 PHYLUM ECHiNODERMATA (Spiny-skinned 

 Animals) 



b. Bilateral symmetry retained throughout life 



(1 ) Without gill slits or internal skeleton 



(a) Without circulatory or excretory 



systems 



PHYLUM CHAETOGNATHA (ArrOW 



Worms) 



(b) With both circulatory and ex- 



cretory systems 

 PHYLUM POGONOPHORA (Beard 

 Worms) 



(2) With gill slits or internal skeleton or 



both 



(a) Without typical notochord (internal 



skeletal rod) in adult or embryo 



PHYLUM HEMICHORDATA (Acom 



Worms) 



(b) Embryo with notochord, gill slits, 



and dorsal hollow nerve cord; 

 adults with gill slits or vertebral 

 column or both 

 PHYLiiM CHORDATA (Chordates) 



ORIGIN OF LIFE 



Because the unifying concept used in treating life 

 is its evolution, a more detailed hypothesis on the 

 origin of life and of the earliest plants and animals 

 must be considered. The hypothesis that is con- 

 sidered here is one favored by many modern 

 biologists, but not all. However, qualifying words 

 or phrases will not be used. 



After the earth first solidified, various materials, 

 including water, were in a gaseous state in the early 

 atmosphere. Atmospheric water fell almost con- 

 stantly as rain, but because of heat from the earth's 

 surface, the raindrops evaporated before reaching the 

 surface. Finally, with additional cooling, the first 

 rain struck the earth's surface and soon the first 

 water came into existence. Owing to almost con- 

 tinuous rainfall and heat, conditions were optimum 

 for the dissolving of materials of the crust and for 

 the reacting of chemical compounds. Present puny 

 efforts of man to duplicate conditions during this 

 phase of earth's history caused the synthesis of amino 

 acids (the building blocks of proteins) and other 

 organic compounds. More recently, one of the es- 



sential building blocks of nucleic acid, genetic ma- 

 terial, was synthesized by a similar experiment. This 

 is important if one remembers that organic com- 

 pounds now are made only by living creatures and 

 are part of the chemicals in living flesh. 



When the earth finally cooled enough to allow the 

 first permanent bodies of water, there were great va- 

 rieties of organic compounds present; the larger of these 

 compounds existed as so-called colloidal solutions in the 

 water (Figure 7.5). Because these solutions included 

 electrically active groups, water accumulated around 

 the surface of individual particles and oppositely 

 charged particles attracted one another. Due to in- 

 creased size, still larger materials settled and heaped 

 up as complex mixtures called coacervales. The co- 

 acervates each absorbed water on their surfaces to 

 form a membrane-like external structure. All of 

 these hypothetical steps indicate the origin of in- 

 dividuality, but not of life itself. 



From such chemically active individuals, the 



organic 



compounds 



colloidal 

 solution 



coQcervote 



virus-like 



archaic moneran 



• 



archaic protistan 



archaic plants 



and 



archaic animals 



Figure 7.5 Hypothetical sequence in the early evolution of life. Plants 

 and onimals most likely had different protistan ancestors. 



