Bridges Programming, an ox, a horse, fossil fuel, humans... what in common?

We will start by discussing what oxen, horses, fossil fuel and humans have in common.  What!  more than you may at first imagine.  Then,  we will consider barter... and were there ever completely barter economies, as we were taught in school, that preceeded the use of currency...?   After this, or before... we will introduce whatever topics seems interesting to us.  And You!  Please feel free to join in.