On Australopithecus as a "beach ape" or "water-adapted ape" and the origin of bipedal gait



Johan Nygren



STORY: The danakil horst was separated from the mainland when the rift zone which is today the Red Sea, and another rift zone which is today the East African Rift, extended into what is today the Afar region, around 6-8 million years ago. The Danakil block then rotated, relative to the Ethiopian coast, because the Afar rift opened up.

There are a few species in the fossil record in Afar that are named, taxonomically, to have been endemic to that region (having only existed there), and the evolution of those species over time, a period of 2 million years, can be read from first older fossils, and then younger fossils which look different.

Australopithecus afarensis , the first human-like apes, and Ardipithecus, an ancestor, is one example. Other examples are Kolpochoerus afarensis, a species of pig, and their ancestors Kolpochoerus deheinzelini.

The trend I look at in the posts below, is that the older ancestral species are found closer to the Ethiopian mainland, and the younger derived species are found to the north-east, following the movement of the Danakil block as it rotated relative to Ethiopia.

The idea then is that those animals were living on the Danakil "micro-continent", along coast-lines that had formed in ways similar to the coast on southern Crete, and that the fossils that are found in Afar have been deposited (via geological processes) from that coastal ecosystem.





Fossil record of a counter-clockwise rotation of Danakil

Within a Danakil-CHLCA, the fossil records of speciation from geographical isolation, with taxa that are endemic to the Danakil region[1] throughout the period of isolation (6.5Ma — 2.9Ma), would follow the position of Danakil throughout the opening of the Afar region.

Within the Kolpochoerus lineage on Danakil, the earlier K. deheinzelini, is found in 4.3 to 4.6 Ma localities at the Aramis Member of Middle Awash[2], where the “Ardi” specimen of Ardipithecus ramidus was found as well, dated to 4.4 Ma. The younger K. afarensis (3.0–3.4 Ma) is found in Hadar[2], where the Lucy specimen of Australopithecus afarensis was found, dated to 3.2 Ma.

Species which could have been endemic to Danakil, and the result of geographical isolation on Danakil, include Hipparion afarense[1], a species of horses that inhabited Danakil, Kolpochoerus afarensis[1], an extinct genus of the pig family, and Hippopotamus afarensis.[1]

References





Tectonic history of Danakil and the origin of Pliocene vertebrate fossils in the Afar region

Open grassland ecosystem by the shore-line, which then collapsed into the Sea of Afar as Danakil migrated in a counter-clockwise rotation, north-east, where most of the vertebrate fauna in fossil sites like Hadar is found in sands and sandstones[1], which originate from Danakil and form brief layers within the lacustrine sediments from the sea.

The geography of Pliocene Danakil could perhaps be similar to the island of Crete, one of the most seismically active areas in the eastern Mediterranean.[2]

That the open grassland on Danakil was deposited by faulting of the higher regions (similar to how the coast-line of Crete has formed), and that those open grasslands in turn were deposited into the Sea of Afar as Danakil rotated counter-clockwise, means that the entire fossil record of the coastal ecosystem on Danakil has been dispersed into Afar, and can be read along fossil sites like Hadar in the Lower Awash, or further south-west at Aramis in the Middle Awash.

The Danakil-origin of the sands and sandstones where vertebrate fossils are found, also means that older species, like Ardipithecus, are found closer to Ethiopia.

References