Saturday, September 02, 2017

5.7 Million Year-Old Human Footprints Found on Greek Island of Crete

Great Googlymoogly!  This one is lighting up all over the Internet.  Fossil footprints have been found in the southern Greek Island of Crete, near the village of Trachilos, that appear to have the distinctive heel-toe-off gait of bipedal humans.  The catch? The prints are 5.7 million years old!  From PhysOrg:
Human feet have a very distinctive shape, different from all other land animals. The combination of a long sole, five short forward-pointing toes without claws, and a hallux ("big toe") that is larger than the other toes, is unique. The feet of our closest relatives, the great apes, look more like a human hand with a thumb-like hallux that sticks out to the side. The Laetoli footprints, thought to have been made by Australopithecus, are quite similar to those of modern humans except that the heel is narrower and the sole lacks a proper arch. By contrast, the 4.4 million year old Ardipithecus ramidus from Ethiopia, the oldest hominin known from reasonably complete fossils, has an ape-like foot. The researchers who described Ardipithecus argued that it is a direct ancestor of later hominins, implying that a human-like foot had not yet evolved at that time.

The new footprints, from Trachilos in western Crete, have an unmistakably human-like form. This is especially true of the toes. The big toe is similar to our own in shape, size and position; it is also associated with a distinct 'ball' on the sole, which is never present in apes. The sole of the foot is proportionately shorter than in the Laetoli prints, but it has the same general form. In short, the shape of the Trachilos prints indicates unambiguously that they belong to an early hominin, somewhat more primitive than the Laetoli trackmaker. They were made on a sandy seashore, possibly a small river delta, whereas the Laetoli tracks were made in volcanic ash.
And now, the other shoe.  How do we know how old the fossil footprints are?  
The coastal rocks at Trachilos, west of Kissamos Harbour in western Crete (Fig. 1a–c), lie within the Platanos Basin, and present a succession of shallow marine late Miocene carbonates and siliciclastics of the Roka Formation, a local development of the Vrysses Group (Freudenthal, 1969 ; van Hinsbergen and Meulenkamp, 2006; Figs. 1d, e and 3a, b). At the top, this marine succession terminates abruptly in the coarse-grained terrigenous sedimentary rocks of the Hellenikon Group (Figs. 1d and 3e, f), which formed by the desiccation of the Mediterranean Basin during the Messinian Salinity Crisis (van Hinsbergen and Meulenkamp, 2006), an event dated to approximately 5.6 Ma (Govers, 2009). The succession (Fig. 1d) contains an emergent horizon with well-preserved terrestrial trace fossils and microbially induced sedimentary structures (Fig. 3d) immediately overlying shallow water ripplemark structures (Fig. 3c).1
So, the authors are a tad more circumspect than the writers of the PhysOrg story.  The authors posit two hypotheses for their results: 1. the tracks represent the gait of a basal hominin, which explains the non-divergence and shape of the big toe as well as the shape of the ends of the other toes, which resemble those of a human foot and not a non-human foot.  This fits approximately with the dates of the north African remains of Orrorin and, perhaps, that of Sahelanthropus (although that is pretty much a surface find).

The Messinian Crisis was a period of time during the Miocene epoch during which the Mediterranean Sea almost completely dried up.  This crisis began around 6 million years ago and ended around 5.3 million years ago with what is known as the Zanclean flood.  It is estimated that once the barrier at the Strait of Gibraltar was broken, the Mediterranean Sea refilled within two years, which means that the sea level rose at an estimated 30 feet per day., lets go back here, to the story that came out about four months ago, establishing the possibility that the last common ancestor to apes and humans was in Europe.  In that study, a jaw with human root patterns and an isolated premolar that have both been attributed to Graecopithecus, were re-examined and found by the researchers to have hominin affinities, a surprising conclusion, given their age of 7.15 million years.  At the time that story appeared, I remarked that it was a bit of a stretch to hang one's hat on one premolar and partial ape-like mandible, but in the context of the new finds, maybe not so much.  This strengthens the (admittedly far-fetched) notion that our ancestors did, in fact, originate somewhere in southeast Mediterranean Europe and, over the course of the next two and half million years ago, migrated south to north Africa.

As Per Ahlberg was quoted as saying:
"This discovery challenges the established narrative of early human evolution head-on and is likely to generate a lot of debate. Whether the human origins research community will accept fossil footprints as conclusive evidence of the presence of hominins in the Miocene of Crete remains to be seen."
This is huge news.  Even if we can't place the LCA in southern Europe, we now have bipedalism extending back into the late Miocene. 

1Gerard D. Gierliński et al, Possible hominin footprints from the late Miocene (c. 5.7 Ma) of Crete?, Proceedings of the Geologists' Association (2017). DOI: 10.1016/j.pgeola.2017.07.006

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