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Atmega pins can source (provide positive current) or sink (provide negative current) up to 40 mA (milliamps) of current to other devices. Also they provide only 5V.

A stepper motor driver like the A4983 provides up to 1A (without a heat sink) or up to 2A (with heat sink) @ up to 35V.

At least you won't get much torque (if any) out of your steppers when driving them directly from the Arduino pins.
I would be more concerned about the possibility to burn out th
e ATMega chip when driving inductive loads like a stepper motor coil directly on the pins. But I'm no expert with this...