When hominins walked in each others' tracks

Archaeologists and paleontologists have been finding more ancient footprint trackways of hominins lately. These discoveries have sped up partly because researchers have learned better how to spot them, and partly because more scientists are seeing how much can be learned from these trace fossils. Many trackways, which would once have been curiosities, are becoming hypothesis tests with the aid of precise 3D data recording methods. 

In the news last week was a trackway and three isolated prints from Koobi Fora, Kenya, around 1.5 million years old, which look to have been made by two different species. The research team that analyzed the prints, led by Kevin Hatala, determined that the trackway was likely made by a large Paranthropus boisei individual, while two or three smaller Homo individuals probably made the isolated prints on the same ancient surface.

I'm really impressed with this work. These trace fossils show something that fossil bones and teeth likely never will: two species of ancient relatives walking on the same few meters of shoreline, maybe within hours of each other. But what surprised me most about the study was that there’s not just one site with two species: There are two. 

Back in time

The newly reported footprint site joins a history of similar discoveries from Koobi Fora. In 1978 Kay Behrensmeyer and Léo Laporte were working on “step trenches” to expose the stratigraphy across a series of sedimentary layers when they noticed “sedimentary structures which, in cross-section, seemed to be imprints of large vertebrate feet on originally soft, muddy substrates”. They exposed some of these ancient surfaces by clearing the overlying sediments, uncovering four-toed tracks of hippos and a few distinctive footprints of hominins. The Koobi Fora footprints were between 1.6 and 1.5 million years old.

Only two years before, in 1976, the first footprints from Laetoli, Tanzania, had been discovered, which would prove to be around 3.6 million years old. The Koobi Fora prints helped show that the Laetoli tracks were not a unique circumstance. There were other footprint sites to be found. 

Already in the 1970s scientists knew that at least two species of hominins had lived in the Koobi Fora area when the trackways were made. Which of these species had made the footprints?

That question remained unanswered when new trackways were uncovered starting in 2005. Near the village of Ileret at a site designated as FwJj14E, fieldworkers found parts of the upper limb skeleton of a hominin, which was later identified by Brian Richmond and coworkers as Paranthropus boisei. Researchers excavated further to outline the geological context of this fossil arm, and they found footprints in the layers below. A series of step trenches revealed two extensive surfaces with footprints. 

This led to an survey for footprint sites in the area. The research team described in several 2016 research articles, including two led by Kevin Hatala and one led by Neil Roach. In one paper, the team reported 97 footprints made by at least 20 different hominins at five sites around Ileret, all around 1.5 million years old.

They also carried out experiments to determine how long the footprints could have retained their shape in the sandy silt without being protected by more sediment. They found that even a day or two of exposure to the elements tended to degrade them. Each of these surfaces is a record of a fairly short time on the landscape.

By far the most extensive data came from the Upper Footprint Layer at FwJj14E, where at least 8 different individuals made trails. Matthew Bennett and coworkers in 2009 focused on a long trail of seven prints, two of which seemed overlapping, like a stutter-step. Later fieldwork exposed a larger surface, showing that this single trail was actually two overlapping trails, made by two different individuals. Hatala and coworkers writing in 2016 designated these two trails as FUT1A and FUT1B, and both were part of a larger group of trackways, most proceeding in a roughly parallel direction. 

The data seemed to open a window onto the behavior of an ancient group of hominins. The prints were relatively large, averaging just over 25 cm in length, with stride lengths up to a meter or more. Considering their size and overall situation, Hatala and collaborators suggested that long-legged H. erectus likely made these prints. The evidence seemed to suggest coordination. Maybe a group of adult male individuals strode with purpose in a common direction.

“At the very least, the evidence from the Ileret track surfaces of multiple H. erectus males walking across the same landscape, and possibly even traveling together, is consistent with a level of male–male cooperation similar to that observed in modern chimpanzees.”—Kevin Hatala and coworkers, 2016

New evidence

The newly-described footprint site was designated as ET-2022-103-FE22, in area 103 of Koobi Fora not far from Behrensmeyer and Laporte's site. The new site, too, has an estimated age in the years just before 1.5 million years ago. The FE22 surface preserves one trackway of prints, and three isolated hominin prints from other individuals. We return again to the old problem: with at least two species in the picture, who made these prints? 

In this case, Hatala and coworkers applied some new methods to assess how well the tracks match those made by modern people. The new prints are deep ones made in soft sandy silt, and the depth affects the outlines of the prints. The team developed a dataset from a large set of human footprints made in different conditions, showing that the form of human footprints has a consistent relationship with the depth of prints. Deep human prints made in soft sandy mud show a high human arch even more consistently than shallow human prints. 

Another thing is that human footprints don’t show the big toe angling very far from the medial side of the print, called abduction. Maybe more important, people today don’t move their big toe much from one footstep to the next. 

Hatala and coworkers found that two of the isolated prints from the new site preserved good evidence of shape and were quite humanlike in their arch height and big toe shape. These, they thought, could be consistent with H. erectus. But the longer HT1 trackway did not fit the range of human footprints. Those prints had a flatter arch, the big toes averaged higher abduction than humans, and showed a lot of variation in abduction angle from step to step. This long trackway, the team concluded, likely was made by a large P. boisei individual.

The prints seem to show that two species with different foot mechanics were both walking on this silty shoreline within a short time of each other. 

I have to admit, when I read the paper I was skeptical at first. The record of fossil foot bones from both these species is fairly sparse. The large HT1 prints suggested a pretty big body size for the P. boisei individual, at an estimated 55 kg it would outweigh the largest known associated P. boisei skeleton. I wondered if we could really be sure that a H. erectus individuals might not have had a flatter foot, or a little bit of splay to the toe. Humans today vary, after all. 

But I was soon persuaded that the differences look real. One yardstick for me is the footprints from Laetoli, Tanzania. Like the HT1 prints, the footprints from the Laetoli G1 and S1 trackways have flatter arches than humans, have big toes more abducted than humans, and abduction angles that vary from step to step. If the Koobi Fora site has some human-looking footprints and other Laetoli-looking footprints, and they are consistently different, I'd say that's about as much as we can ask of trace fossils to give us evidence of phylogenetic diversity. It's a good hypothesis.

But if this new site has P. boisei footprints, what about the others? The conclusion to the paper mentions a reanalysis of the FwJj14E site, but I had to look carefully at the supplementary information of the paper to find out the conclusion. Using their new approach, Hatala and coworkers revisited FwJj14E and some of the other footprint sites. Some of those prints were incompatible with human footprints, too. 

With the new data, the Upper Footprint Layer trails now include not only H. erectus but also at least one P. boisei trackway. The most interesting example is the one discussed above, the FUT1 trail that in 2016 Hatala and collaborators showed was actually two overlapping trackways. Now the footprints themselves support the reinterpretation that these two overlapping trackways were made by two different species: the FUT1A trackway by H. erectus and the FUT1B trackway by P. boisei. Unlike the Koobi Fora example, where the H. erectus prints are the isolated, at the FwJj14E site two species made long trackways in the same direction on the same path.

To me, that was a surprising twist with greater import than the new footprint site. This reinterpretation of the Ileret tracks contradicts the ideas from 2016 about a pack of male Homo erectus individuals moving together—at least, unless the pack included a boisei member. Maybe instead, a smaller erectus group might have followed one or more boisei individuals down the shoreline. Or maybe they were foraging in the same place for the same foods.

Finding interactions

So much our work goes toward finding some way to glimpse those fleeting moments when individuals, populations, or species touched each other in some way. 

The growing DNA record of the last 15 years has brought again and again the interactions between some populations into view. The Neanderthals, Denisovans, and contemporary humans within Africa were made up of populations that developed in partial isolation from each other with occasional interactions and mixture. When such groups met they found ways to coexist, at least for a time. They shared each other’s space, some had children together, and many of their descendants made their own space in the world. 

Could more distant relatives like Paranthropus eventually come into view? Proteomic research has already started to reveal some of the diversity of Paranthropus robustus. The latest fossils currently identified as Paranthropus are in the range of 800,000 to 1.2 million years old, from Swartkrans, South Africa. That is still a lot older than any DNA evidence from African sites. But I doubt we’ve yet found the last of Paranthropus. The diversity of earlier Middle Pleistocene African hominins is clearly undersampled, and Paranthropus was a survivor. There was no obvious reason for these hominins to have become extinct at the onset of the Middle Pleistocene. I expect even younger evidence of this group will emerge. With advances in technology, maybe Paranthropus genomes will come into reach. 

The sample of footprint surfaces is growing, and two of the most extensive surfaces from this time and place were walked upon by two hominin species. The Laetoli surfaces, too, seem to have different kinds of hominins represented across them, two million years before the time of the Koobi Fora and Ileret trackways. What the footprints help to show is that different human relatives were inhabiting the same spaces when they lived. That's much more than saying that they sometimes met.

Notes: The references on the Ileret footprint trackways from the last fifteen years have really expanded the kinds of evidence that can be gathered from these ancient trace fossils. When you compare the work that was happening in 2007 to what is happening now, it's quite remarkable. I've included several of those references here, some are open access, and they are worth reviewing for those interested in learning more.

In this post I use the name Homo erectus as I often do, where other scientists might prefer Homo ergaster or even some other name. I've had several readers ask me about the classification of these fossils lately. More on the way.

Looking in retrospect from the most recent study, I expect that there must be more work to align footprints and skeletal evidence. As I was writing about this research, I went down a side quest to update myself on foot fossils from the various species. There are a lot more of them than there used to be, and the excellent review by Jeremy DeSilva and coworkers cited below is a good summary. Still it's fair to say that the fossil record of the foot of H. erectus and P. boisei does not yet tell us enough about the variation in either species.

References

Behrensmeyer, A. K., & Laporte, L. F. (1981). Footprints of a Pleistocene hominid in northern Kenya. Nature, 289(5794), 167–169. https://doi.org/10.1038/289167a0

Bennett, M. R., Falkingham, P., Morse, S. A., Bates, K., & Crompton, R. H. (2013). Preserving the Impossible: Conservation of Soft-Sediment Hominin Footprint Sites and Strategies for Three-Dimensional Digital Data Capture. PLOS ONE, 8(4), e60755. https://doi.org/10.1371/journal.pone.0060755

Bennett, M. R., Harris, J. W. K., Richmond, B. G., Braun, D. R., Mbua, E., Kiura, P., Olago, D., Kibunjia, M., Omuombo, C., Behrensmeyer, A. K., Huddart, D., & Gonzalez, S. (2009). Early Hominin Foot Morphology Based on 1.5-Million-Year-Old Footprints from Ileret, Kenya. Science, 323(5918), 1197–1201. https://doi.org/10.1126/science.1168132

DeSilva, J., McNutt, E., Benoit, J., & Zipfel, B. (2019). One small step: A review of Plio-Pleistocene hominin foot evolution. American Journal of Physical Anthropology, 168, 63–140. Scopus. https://doi.org/10.1002/ajpa.23750

Hatala, K. G., Roach, N. T., Behrensmeyer, A. K., Falkingham, P. L., Gatesy, S. M., Williams-Hatala, E. M., Feibel, C. S., Dalacha, I., Kirinya, M., Linga, E., Loki, R., Alkoro, A., Longaye, Longaye, M., Lonyericho, E., Loyapan, I., Nakudo, N., Nyete, C., & Leakey, L. N. (2024). Footprint evidence for locomotor diversity and shared habitats among early Pleistocene hominins. Science, 386(6725), 1004–1010. https://doi.org/10.1126/science.ado5275

Hatala, K. G., Roach, N. T., Ostrofsky, K. R., Wunderlich, R. E., Dingwall, H. L., Villmoare, B. A., Green, D. J., Braun, D. R., Harris, J. W. K., Behrensmeyer, A. K., & Richmond, B. G. (2017). Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma. Journal of Human Evolution, 112, 93–104. https://doi.org/10.1016/j.jhevol.2017.08.013

Hatala, K. G., Roach, N. T., Ostrofsky, K. R., Wunderlich, R. E., Dingwall, H. L., Villmoare, B. A., Green, D. J., Harris, J. W. K., Braun, D. R., & Richmond, B. G. (2016). Footprints reveal direct evidence of group behavior and locomotion in Homo erectus. Scientific Reports, 6(1), 28766. https://doi.org/10.1038/srep28766

Richmond, B. G., Green, D. J., Lague, M. R., Chirchir, H., Behrensmeyer, A. K., Bobe, R., Bamford, M. K., Griffin, N. L., Gunz, P., Mbua, E., Merritt, S. R., Pobiner, B., Kiura, P., Kibunjia, M., Harris, J. W. K., & Braun, D. R. (2020). The upper limb of Paranthropus boisei from Ileret, Kenya. Journal of Human Evolution, 141, 102727. https://doi.org/10.1016/j.jhevol.2019.102727

Roach, N. T., Hatala, K. G., Ostrofsky, K. R., Villmoare, B., Reeves, J. S., Du, A., Braun, D. R., Harris, J. W. K., Behrensmeyer, A. K., & Richmond, B. G. (2016). Pleistocene footprints show intensive use of lake margin habitats by Homo erectus groups. Scientific Reports, 6(1), 26374. https://doi.org/10.1038/srep26374