Final answer:
The key hominin fossil discovery announced in 2015 was Homo naledi, which exhibited a mix of primitive and modern traits and had striking similarities to modern humans in terms of bipedal locomotion.
Step-by-step explanation:
Key Hominin Fossil Discoveries Announced in 2015
The key hominin fossil discovery announced in 2015 was Homo naledi. This species, unearthed in the Rising Star cave system in South Africa, presented a mix of primitive and modern traits. Interestingly, Homo naledi had a small brain size of approximately 560 cubic centimeters which is similar to that of the australopiths. Despite their small cranial capacity, they were well adapted for upright walking, and this, along with the shape of their feet, showed strong similarities to modern humans.
Other species such as Homo habilis, Australopithecus afarensis, and Homo neanderthalensis have been well-documented in earlier discoveries. Homo habilis, often referred to as "handy man," was an early species in the Homo genus and is considered a possible ancestor of later species like Homo erectus. Australopithecus afarensis is a well-known early hominin genus and was contemporaneous with early Homo species. The Neanderthals (Homo neanderthalensis), being much later, were not part of the 2015 announcements and have been recognized for a longer period.