Ardipithecus kadabba

The oldest known Ardipithecus; kadabba

Haile-Selassie, Y. Late Miocene hominids from the Middle Awash, Ethiopia. Nature 412, 178–181 (2001).

The split from Ar. Ramidus

While Ardipithecus kadabba fossils were found well before 2004, it was not until kadabba's type specimen ALA-VP-2/10 was found that Ardipithecus kadabba became its own species. The morphologies significantly differed enough to spur an evaluation of whose fossils they really were. This split from Ramidus brought with it 17 specimens!

Discovery!

Tim White December 27, 2000

Tim White December 16, 2000

The right image shows the Southwest view over the Middle Awash region's ALA-VP 2 area. The image was actually taken the day of A. ramidus' discovery. Note the rocky soil projections in the landscape, making it a fantastic location to scour the ground for fossils. Once a specimen is discovered, its location is immediately recorded, which is what we can see happening in the left image, also in the Middle Awash region.

Ardipithecus kadabba. (2022, June 30). The Smithsonian Institution's Human Origins Program.

Paleoenvironment

Contrary to what most people would believe, our hominin ancestors, especially Ardi. kadabba lived in water-rich environments not the savannah dominating Africa today. For Kadabba in the late Miocene period, floodplains, large aquatic habitats, either lakes or rivers, and woodland surrounding them would have been a common sight ( Haile-Selassie & WoldeGabriel, 2023)

Haile-Selassie, Y. Late Miocene hominids from the Middle Awash, Ethiopia. Nature 412, 178–181 (2001).

Kadabba's Type Specimen.

ALA-VP-2/10's teeth (section a of the figure) suggested a new species from Ardipithecus ramidus. Ar. ramidus' teeth did not have a honing complex while this specimen did. The lower canines specifically show posterior wear typical of a honing complex (Haile-Selaisse, 2001). This wear is where the canine would rub and sharpen its shape as the jaws closed. Additionally, other ramidus specimens showed Honing, thin enamel, curved phalanges, climbing humeri, and a bipedal toe.

Haile-Selassie, Y., & WoldeGabriel, G. Ardipithecus kadabba: Late Miocene evidence from the middle awash, Ethiopia. University of California Press (2023).

A better look

To better showcase Kadabba's honing complex, look no further than specimen ASK VP 3/400. Rows A-C show micro-CTs of the upper right canine, and row D shows the lower right canine. The image clearly shows the posterior wear on the lower right canine as the upper right canine conversely displays the anterior wear sloping inward. This highly suggests a honing complex for the species. These canines were so striking that one of them even stars as the cover image of Haile-Selassie's book on Kadabba!

Fun Fact

ASK-VP-3/78 has arthritis on the trochlea which is not commonly seen on older hominins. Blowup B in the figure especially accentuates the arthritic subchondral erosion as the matrix below the surface is exposed.

Why is this not commonly seen?

For one reason, arthritis is a degenerative disease that is more common as you age, and the early hominins likely did not live that long compared to modern humans. Arthritis is also a function of wear from weight bearing, so as humans became more bipedal the more weight is placed onto select joints, increasing arthritic likelihood. However, ASK-VP-3/78's arthritis is actually on the Humerus making it even more peculiar. Could it have been autoimmune in nature? Sadly, we will likely never know with our current evidence.

Haile-Selassie, Y., & WoldeGabriel, G. Ardipithecus kadabba: Late Miocene evidence from the middle awash, Ethiopia. University of California Press (2023).

Biped toe explained

Kadabba's special bipedal toe can be seen in specimen AME-VP-1/71, and its morphologies are hard to wrap your head around. However, it can be seen that the left fourth foot phalanx is angled up (in this case, dorsally on the foot) and inward. This suggests dorsiflexion impact as seen in bipeds (Haile-Selaisse, 2001).

Visually and simply, walking bipedally requires increased dorsiflexion (the rightmost image), requiring the phalange to change in morphology and increase the articulation angle (leftmost image).

March 2014 – Lawnchair anthropology. (2014). Lawnchair Anthropology. https://lawnchairanthropology.com/2014/03/

But why was kadabba not put after ramidus, why before?

This distinction was under debate once kadabba became its own species, so an assessment and comparison of primitive features were made. ARA-VP-7/2's ulna has some of the morphologies that helped distinguish the species. Specifically, while the arm bones are overall significantly smaller than ramidus, it is the ulna's anterior concavity that is seen only in older species like Orrorin tugenensis and Sahelanthropus tchadensis. ramidus lacks this feature and the honing complex which is also a primitive feature, solidifying ramidus as the more ancestral species.

Given this, re-inspecting ramidus should give morphologies that kadabba passed down. Some of the morphologies that pointed to ancestry were the inclusion of kadabba's ancestral thin enamel, climbing humeri, and curved phalanges in ramidus' morphology. These climbing morphologies, with the similarity of the fossils in the same location, all came together to piece together an idea of what the lineage may have been (Welker, 2017).

Wong, J., & Hendry, L. (2015, December 13). The origin of our species.

The new progenitor

The species now sits comfortably at an estimated 5.8-5.2 million years old. This places it just under Orrorin tugenensis, and the oldest Ardipithecus. However, do not be too quick to place kadabba as one of modern humans' oldest ancestors as while it is probable, we just don't have enough fossil evidence to safely say.

Citations

Haile-Selassie, Y., & WoldeGabriel, G. (2023). Ardipithecus kadabba: Late Miocene evidence from the middle awash, Ethiopia.

University of California Press.

Haile-Selassie, Y. Late Miocene hominids from the Middle Awash, Ethiopia. Nature 412, 178–181 (2001). https://

doi.org/10.1038/35084063

MacLatchy, Laura & Desilva, Jeremy & Sanders, William & Wood, Bernard. (2010). Hominini. Cenozoic Mammals of Africa. 471-540.

10.1525/california/9780520257214.003.0025.

Welker, B. H. (2017, June 13). 8. Ardipithecus ramidus, Ardipithecus kadabba – The history of our Tribe: Hominini. Milne Publishing –

Publishing at Milne Library SUNY Geneseo.

https://milnepublishing.geneseo.edu/the-history-of-our-tribe-hominini/chapter/ardipithecus-ramidus-ardipithecus-kadabba/

Wong, J., & Hendry, L. (2015, December 13). The origin of our species. https://www.nhm.ac.uk/discover/the-origin-of-our-species.html

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