"Bones speak, but they do not enunciate." -Priscilla Long (Long, 2005)

Who was Orrorin tugenensis?

Orrorin tugenensis is one of the earliest known hominin species. Recovered fossils date to around six million years ago--so far back in time that they may have lived at the same time as the very earliest known hominin ancestor, Sahelanthropus tchadensis. 

"Orrorin tugenensis" means "original man from Tugen", which is the name given to the species by Martin Pickford and Brigitte Senut, the paleoanthropology team leaders who discovered and identified it in 2001. Pickford and Senut chose this name in part because O. tugenesis fossils were discovered in the Tugen Hills, Kenya, in the Rift Valley. The "original man" part of the name has a more complicated explanation. 

What Makes the Discovery of Orrorin tugenensis Unique?

Senut and Pickford not only discovered a new species, but an entirely new genus. How this new genus fits into the grand phylogentic scheme is not resolved, but Senut and Pickford list descriptions and comparisons for the species:

"Order Primates Linnæus, 1758
Suborder Anthropoidea Mivart, 1864
Superfamily Hominoidea Gray, 1825
Family Hominidae Gray, 1825
Genus Orrorin Senut et al., 2001
Orrorin tugenensis Senut et al., 2001"
(Senut, Pickford, et al, 2001) 

Morphology

Orrorin tugenesis is a very early hominid with very few fossils surviving to clue us in on what it may have looked like. Nonetheless, paleoanthropologists have been able to determine some important traits that distinguish it from other hominids and hominins. 

Orrorin tugenesis's dentition is a great place to observe some of these differences. O. tugenesis had apelike dentition--incisors, canines and premolars--and small molars in comparison to its front teeth. A trait that defies an otherwise ancestral-looking mouth of teeth, is the derived trait of having thick enamel. 

Like its ancestors, Orrorin tugenesis likely spent a lot of time in the trees. The recovered phalanx fossils are curved, which would have given it the ability to easily hang from branches. This is especially true when paired with the shape of humerus (upper arm bone) found, which is adapted to climbing.

Though it is contentious, there is indication in the shape of Orrorin tugenesis's femur that it had the ability to walk on two legs at least some of the time. The round/spherical head is rotated at a slight angle that would facilitate 2-legged walking. Another clue is the length of the femoral neck. This is important because there are researchers who believe Orrorin tugenesis was bipedal often enough to be considered the first hominin. Others are not convinced. For example, there are some who strongly argue the cortical bone distribution point to bipedalism.  (Davers et all, 2022) There are those who argue compellingly that there is not enough evidence for anything more than infrequent bipedalism at most. (Cazenave, Pina, et al, 2024) 

Beyond these traits, there simply is not enough fossil material to make a reasonably educated guess about how  Orrorin tugenesis may have looked, behaved, or how it was related to other hominids and hominins.

Hominid vs. Hominin: The Difference a Letter Makes!

Was Orrorin tugenensis was a hominid or a hominin? This extensively-argued question hinges entirely on whether or not O. tugenensis was bipedal and therefore considered a hominin. Some scientists do not believe there is enough evidence to conclude O. tugenensis was bipedal. On the other end of the spectrum, there are those who believe O. tugenensis was not only a bipedal hominin, but a direct ancestor to the obligate bipedal genus Homo. The latter hypothesis requires a radical re-configuration of the phylogenetic tree, moving australopithecines to their own branch. The team who discovered and identified Orrorin tugenensis advocate for this model. 

Some researchers fall on the side of Orrorin tugenesis being a biped based on femur morphology. When exploring evidence for bipedalism in Miocene apes, G. Davers and colleagues note how some Miocene apes' femurs, including Orrorin tugenesis's, have "... proximal platymeria immediately distal to the lesser trochanter ... a trait observed in O. tugenensis and later hominins and suggests a correlation with neck elongation, an indicator of hominin bipedalism" (Davers et all, 2022). Davers also links the femur's cortical thickness and the presence of calcar femorale, all found in O. tugenensis, with bipedalism. 

Marine Cazenave and colleagues have independently tested Davers et al's findings and have disagreed with their conclusion on nearly every point. For example, Cazenave has determined O. tugenesis's femur's cortical thickness and distribution, which the Davers publication associated with bipedalism, is similar to Pongo (chimpanzee) and therefore cannot be used as evidence for habitual bipedalism. (Cazenave, Pina, et al, 2024) 

Orrorin tugenensis's status as a hominin and position on the the phylogenetic tree is still debated. 

So Where is Orrorin tugenensis on the Evolutionary Tree? 

Why does it matter so much if Orrorin tugenesis was a hominin or a hominid? Does it really make a difference to science if it is an ancestor to Homo? There are many disagreements about ancient hominin phylogeny, but the answer to this question has particularly impactful ramifications. If Orrorin tugenesis is Homo's ancestor, the model for how we order and understand hominin lineage is radically reconfigured. In fact, the australopithecines, who we currently believe are Homo's ancestor, would be relegated to a dead-end branch of the phylogenetic tree. 

Fossil Specimens