John McCulloch died as a result of the blaze at West Barmoss Avenue, Port Glasgow, in 2009.
Martin Olej, 22, and Iain Starrett, 21, went on trial accused of murdering the 69-year-old by setting the blaze.
Pathologist Marjorie Turner was the first witness heard at the High Court in Glasgow yesterday.
Ms Turner said charring of the corpse, images of which were shown on screens in the court, was so bad it could have "obliterated" any possible injuries.
Olej, of Bearsden, and Starrett, of Port Glasgow, are said to have assaulted Mr McCulloch then lit the fire with lighter fuel.