ABSTRACT
We k n ow tha t al l l iving organisms ar e composed o f cell s a nd cell products, tha t all
cells aris e fro m othe r cells , an d tha t al l organism s hav e arise n fro m a commo n
primordial ancesto r b y evolutionar y diversification . Severa l different evolution -
ary scenarios have been proposed , an d each has its proponents. A l thoug h it is no t
possible t o mak e a n absolut e choic e o n th e base s o f curren t biochemica l a n d
morphologic data , we hav e arbitraril y selected th e schem e s h o w n i n Figure 1.1. I n
this scheme , th e putativ e an d undefine d commo n ancesto r i s believe d t o b e fa r
less comple x tha n an y k n o w n prokaryotic organism (cell s withou t a membrane -
bound nucleus) . Thi s commo n ancesto r i s presume d t o hav e diverge d t o yie l d
three differen t prokaryoti c lines. Th e archaebacteria and eubacteria (true bacteria)
kept th e prokaryoti c pattern o f cellula r organization, wherea s eukaryotic organ -
isms (cell s wi t h a membrane-boun d nucleus ) evolve d fro m a thir d prokaryoti c
ancestor an d develope d a compartmentalize d cellula r structure. Archaebacteria
are amazing organisms: they thriv e in the mos t hostil e o f environments, fo r exam -
ple unde r extreme sal t conditions (halophilous) , ho t aci d conditions (sulfu r bacte-
ria), bogs , wher e the y reduc e C 0 2 t o methan e (methanogens) , an d a t extrem e
ocean depths .
