Some residents of northeastern India are growing bridges.  The bridges are formed by diverting above-ground roots of rubber trees.  The roots are directed from the tree to cross a stream, where they land in soil and anchor the living structure.  The resulting pedestrian bridges are formed as the roots are shaped and supplemental planks, flat stones and structures are added to improve passage.

Living bridges take a lot of patience to grow.  As many as ten years or more are needed before a new crossing is usable.  Such bridges may be the ultimate in sustainable design and construction.  Along with patience comes a type of long-range planning and appreciation of place.  Some of the present bridges may have been in use for 500 years.  

From our engineering history, we learn that all bridge types have old antecedents.  Before modern suspension bridges, for example, came old vine rope bridges.  Standard beam bridges are predated by clapper bridges and tree trunks falling in the forest.  As of now there are no modern structures that trace their form to the living bridges of India.  Maybe future bioengineering processes will develop living replacements for steel and concrete that can grow into bridges.