spontaneous

If something is spontaneous, it happens without any outside cause; nothing else makes it happen.

Example: He made a spontaneous offer to help the old lady cross the street.

If something is spontaneous, it is random.

If something is spontaneous, it happens suddenly; it happens without any warning.

If someone does something in a spontaneous way, it is done of their own free will or without making any plan to do so.

If something is spontaneous, it was made without being planted by humans; it is indigenous.