Background

• Since the 1970s several prints and trails of mammal, bird and insect have been identified in 18 sites (labelled from A to R) out of 33 total palaeontological localities in the Laetoli area, Tanzania.
• In 1978 a 27-meter footprint trail was found at Site G, with about 70 footprints corresponding to 3 hominins.  They were bipedal, had big toes in line with the rest of their foot, and their gait was «heel-strike» followed by «toe-off», that is, the same way modern humans walk.
• The footprints were ascribed to Australopithecus afarensis, as suggested by the dating (3.66 Ma) and the fossils found nearby in the same sediment layer.

The new find

• Site S is located only 150 m away from Site G. In October 2014 some excavation works were executed to assess the impact of building a museum including a protective covering for the Site G tracks. This yielded 14 hominin tracks plus other 529 tracks left by other animals including bovids, equids, girafs, rhinos…

Figure 7 from Masao et al. 2016. Original caption: Southern part of the hominin trackway in test-pit L8. Footprints L8/S1-1, L8/S1-2, L8/S1-3 and L8/S1-4 are visible from left to right. The heel drag mark is well visible posteriorly to L8/S1-3.

• 13 tracks belonging to the same indidivual were left along 32 m. 1 other track belongs to a second individual.
• The geological study shows that the trackways of Site G and Site S are contemporaneous. The sediments were deposited by distinct eruptions closely spaced in time, estimated in weeks. This imply that all the tracks belong to the same general population of hominins.
• Both sets of prints were made by individuals walking in the same north-westerly direction, on the same surface, at a similar speed… and during a single trip? Are there more tracks between Site G and Site S?

Body size and dimorphism

• The 3 individuals from Site G had different body size: G1 was the smallest one and walked side by side on the left of the largest one G2, while the intermediate-sized one G3, superimposed its feet over those of G2.
• The overall morphology of the S1 tracks matches those at Site G and is similar in particular to the prints of the larger individual G2.
• The average stature and body mass estimated for S1 is 161-168 cm, 41-48 kg, larger than any other Australopithecus in the fossil record (and hence initially nicknamed ‘Chewie’ after the 2-metre-tall Star Wars character Chewbacca).
• Estimates for S2: 142-149 cm, 37-42 kg.
• Considering S1 as a possible male requires that we reconsider the sex and age of the other Laetoli individuals. A possible tentative conclusion is: S1 male; G2 and S2 females, G1 and G3 smaller females or juvenile individuals.

The tallest and smallest fossil hominins on record – by species – between 1 and 4 mya. Photo credit: Marco-Cherin. Nature

More information:

• Masao F, Ichumbaki EB, Cherin M, Barili A, Boschian G, Iurino DA, Menconero S, Moggi-Cecchi J, Manzi G. 2016. New footprints from Laetoli (Tanzania) provide evidence for marked body size variation in early hominins. eLife 2016;5:e19568. doi:10.7554/eLife.19568 
• The oldest human footprints by continent | Nutcracker Man (link)