This paper (abstract below) describes Danuvius guggenmosi from the Miocene of Bavaria, 11.62 million years old, around the time great apes started to diverge into lineages which would give rise to gorillas, chimps, humans, and bonobos. The type of locomotion it used is claimed to be a modified form of bipedalism which they call “extended limb clambering.” This would lend a bit more credibility to the idea that quadrupedalism is a derived trait in non-human great apes. It is hot off the presses, and this isn’t the first time such a thing has been claimed, but exciting nonetheless.
https://www.nature.com/articles/s41586-019-1731-0
Update: Full paper:
https://www.nature.com/articles/s41586-019-1731-0.epdf?referrer_access_token=VDVF2mJ0lVofld8_madjttRgN0jAjWel9jnR3ZoTv0MpMJV7uq1pC2z9TlFLcopWAwsUutKmnIQkQ9UatmGBhFbbK0TqgHY6DOdEwLF7zxg9jcVJzhHgeUec4SXds2t2K54ZcgXJyXyUChehzfQs_nuIO6zLpD5p57osl9HmfIS4CCPmGYQlMcB75-PqvezwQ90kw_MMZRjrQzwrHBa8hpfgpIdXBMsjkAHpBtdH3fgRz0TPA3HiaoFlaXKL4BFUnxXrdJAYVqTlkYjuSHobfCkpECcbjdsp0qnRPEHkBeiR1woxUL-dPJxf9Cc7x8sXO-FNv0I9g7MraGFHHLfT33QN8WUElO7bdoRZOKHUUhw2PVnBJNDGV3WPQnxDhxY5FFg5xP8VmTVeP72XSU-_5A%3D%3D&tracking_referrer=www.smithsonianmag.com