Human touch pins on MCU (atmega328p) directly or other components needed?
There are two approaches that you can use that don't require much in the way of extra circuitry or processing.  The easiest is to  use a pair of electrodes complete a circuit to provide base current to a transistor or darlington pair, and connect the emitter to the Atmel pin.  You could then configure the pin as an external interrupt (or maybe a comparator), or simply analog sample it, depending on what you want to do. A good description of your options is at http://www.electronicshub.org/touch-sensors/, the source of the images.





The second approach would be to build a capacitive sensor.  You could use a megaohm range resistor, and just connect one end to the pin and the other to the air, hanging free.  When the free end is touched, the noise floor will raise.  You would need to sample pretty fast and process it, as well as doing some analog conditioning to make the signal suitable for single-ended acquisition, and you might need some gain.  There are chips that do this for you that other people have pointed to that will do this job better than you and a microcontroller can achieve on your own.