Take a look at the first 1040. The form and instructions are only four pages long.
The first Form 1040 was produced in 1913 after the 16th Amendment was ratified. The amendment said,
The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration.
The 1913 Form 1040 was three pages long, and it was accompanied by one page of instructions. The normal tax rate was one percent.
The normal tax of 1 per cent shall be assessed on the total net income less the specific exemption of $3,000 or $4,000 as the case may be. (For the year 1913, the specific exemption allowable is $2,500 or $3,333.33, as the case may be.)
There was also an additional or super tax on taxable income above $20,000.