Bitcoin still faces many challenges in becoming a globally accepted currency, Bank of America Merrill Lynch’s commodity and derivatives strategist Francisco Blanch said in a sweeping Monday report comparing digital currencies’ growth to that of gold, silver and even salt in ancient times.

“A crucial hurdle” for bitcoin is whether major financial institutions will accept the digital currency as collateral, said Blanch, who is considered by many to be one of the best commodity analysts on Wall Street. “But we are not aware of any major institution that takes cryptocurrency as collateral at the moment.”

The report is one of a handful from major Wall Street firms and strategists that have recently warmed to bitcoin for its potential as an investment product. The digital currency briefly tripled in value this year, rising from below $1,000 to $3,000 in June. Bitcoin traded near $2,800 Monday.

Beginning with salt in ancient times, Blanch noted how practical needs and new discoveries have led to the development of new currencies: