56 Life: Its Nature and Origin 



known for some years that in these organisms the haploid cells are 

 of two types identical morphologically and metabolically but with 

 different mating reactions. Cells of like type will not mate with 

 each other but only with cells of the opposite type. Experiments 

 with surface solvents suggest very strongly that each type has a 

 distinctive surface chemical and that the two kinds of chemicals 

 form a loose bonding with each other. The surface compound of 

 one haploid type is believed to be a protein, that of the other a 

 polysaccharide, two compounds which would theoretically unite 

 by weak hydrogen bonds ( Brock, 1959 ) . \Miatever the exact nature 

 of the chemicals, when two haploid cells of opposite type come 

 in contact, an agglutination occurs embracing the touching surfaces 

 and holding the two cells together. After the two haploid cells 

 agglutinate, they unite to form a diploid cell. 



However these things came about, eventually there did evolve 

 living organisms having the properties of our idealized cell and 

 having some kind of a sexual mechanism by which characteristics 

 of one parent could be combined with those of another. 



The Time Scale 



It is difficult to date the events outlined in this and the preceding 

 chapter accurately. Cosmologists use data associated with nuclear 

 reactions and spectrographic analysis of heavenly bodies to arrive 

 at ages for celestial events. Geologists and physicists use isotopes 

 of radioactive elements in rocks and sediments to date early periods 

 of the earth's history (Aldrich, 1956). Certain isotopic ratios in 

 elements may also give clues to some past environmental condi- 

 tions (Lowenstam and Epstein, 1954; Emiliani, 1955). 



Life as we know it did not appear until the earth's crust had 

 formed and its climate had reached a stage approximately like 

 that prevailing today. Also life must have formed during the period 

 when the atmosphere was in its non-oxidizing condition, with no 

 free oxygen. This period is estimated to have occurred more than 

 two billion years ago. The fossil record proves that life had become 

 highly diverse as early as the pre-Cambrian era, at least 600,000,000 

 years ago (Glaessner, 1961). Weighing these factors, it seems 

 probable that life originated at least two billion years ago. 



Summarizing the best-founded estimates and calculations gives 

 the following chronology for the e\ents which occurred from the 

 beginning of the universe to the first fossil beds of the Paleozoic era: 



