The basic 2x4 brick has many versions. For example the modern one has thinner walls and funny shaped tubes compared to an early 1980s brick. What were the changes over the years and how did they affect building?
I can't provide a list of all the minor changes, but I can at least tell the most significant change made in the history of the basic 2x4 brick (which applies to all other bricks and parts, too):
In general, there are no essential changes made: a brand-new brick still fits together with one from before 1963 as long as the Cellulose Acetate hasn't deformed too much during ageing (which is the main reason they switched to ABS, it's more robust, scratch-proof and doesn't deform so easily).
The main "change" that really affects building is the continuously improving precision in production - this happens all the time, so there's no point on the timeline. The effect is simple: better fitting bricks improve the stability of your construction.
Hollow tube 1958 (2×4 weighing 3g)
Gets a horizontal ridge called a flowrib & end pips, holds together better (now 2•68g)
Colours charge & black shine, maybe from being made from ABS instead of CA?
Flowrib removed (= weight)
In USA gets slits in tubes
As Pat. Pend. ends, those words get obscured, & stud pips instead of side pips 1974
2×3 & 2×4 get cross-members, ribs & thinner walls (2•25g) (sorry I don't get the joke? tubes still round). [2×2 gets ribs]
The N° for 2×4, 3001, inside those bricks.
& for the 1×
Gets pillars (not for 1×1), 1×6 now has 1 cross-member instead of 2 1959
Gets full depth cross-members, & 1×6 back to having 2 (but there is also some with just the 1)
Cross-members on all pillars, & not full depth again
The 1st using tubes, & 1× sans pillars don't hold together well, okay for buildings. Must be bigger under & smaller studs? Some one wrote that the new thinner bricks don't hold as well on underside. I've 1×2 with notch for wires, hold together okay, even though no pillar, but guess the pillars must help.
With cross-members & ribs, when I tested using bricks that came just before, holds less well underneath, using 6 studs there holds less well than 4 atop. But better with the 1st using tubes, 4 underneath better than 8 atop. But no better at holding 1 stud under.