Changes

*2.  Density waves have been proposed as a possible explanation for this.  DW are like sound waves; areas of compression and areas of rarefaction.  Stars move in and out of these waves (as do air molecules in sound waves).  These density waves would also kick off star formation in there dense regions (also explaining why there are so many supernovae in the arms; there are bigger stars there).

*2.  Density waves have been proposed as a possible explanation for this.  DW are like sound waves; areas of compression and areas of rarefaction.  Stars move in and out of these waves (as do air molecules in sound waves).  These density waves would also kick off star formation in there dense regions (also explaining why there are so many supernovae in the arms; there are bigger stars there).

*3.+4.  You really need agriculture to have civilization, as it support larger population with fewer people, so those left over can do useful things like live in cities, rule over others, etc.  Some have suggested that writing was started for tax purposes (the ruler wants all he can get).  The modern reader has to appreciate the fact that even conceiving of writing is a huge step in culture: the Incas didn't have it (and they were around for thousands of years).  For agriculture, one could draw the analogy with moveable type for the printing press; it seems obvious now, but it took a while to come up with.  While ancient man had all of the intelligence, it had none of the advances in technology and learning (all of the hardware, none of the software).

*3.+4.  You really need agriculture to have civilization, as it support larger population with fewer people, so those left over can do useful things like live in cities, rule over others, etc.  Some have suggested that writing was started for tax purposes (the ruler wants all he can get).  The modern reader has to appreciate the fact that even conceiving of writing is a huge step in culture: the Incas didn't have it (and they were around for thousands of years).  For agriculture, one could draw the analogy with moveable type for the printing press; it seems obvious now, but it took a while to come up with.  While ancient man had all of the intelligence, it had none of the advances in technology and learning (all of the hardware, none of the software).

==Science==

==Science==