We now consider the evidence which science has pro- 

 duced regarding the origination of living beings on the 

 Earth. Firstly, there is the conclusive evidence of the 

 fossils of many different types of plants and animals found 

 in geological strata. These have shown that new species 

 have arisen successively from older ones, the most recent 

 fossils being found in the topmost strata, and the earliest 

 fossils in the lowest strata. The absolute ages of the different 

 strata are determined by the physical method of radio- 

 activity dating, which is independent of any environmental 

 factors except for the time elapsed since the strata were 

 laid down as sediments in shallow waters. The process 

 of evolution of living organisms is thus found to take 

 place as a slow variation in their form during the periods 

 of millions of years for which life has been present on 

 Earth. 



The oldest known living things have the simplest struc- 

 tures, and are dated to greater than 600 million years ago, 

 in the Pre-Cambrian period. These were seaweeds which 

 have left impressions of their forms in the rocks, but in 

 older rocks indications of life are still found in traces left 

 by creatures crawling over ancient mud. (Several Pre- 

 Cambrian invertebrate animal fossils have been found 

 recently in Australia). It is evident that most of the Pre- 

 Cambrian life must have been soft-bodied, and consequently 

 left little traces of its existence in the rocks after death. This 

 is what we would expect, since the simplest living types 

 of the present-day are without hard parts (e.g. the proto- 

 zoa, polyzoa, etc.). Thus life must have originated some- 

 time in the Pre-Cambrian, most probably in the liquid 

 environment of the seas, during the long intervals of warm 

 climate which have existed between the widely spaced 

 Ice Ages of the past. The actual age of the Earth is today 



54 



