What I found most interesting while reading Papagianni & Morse’s The Neanderthals
Rediscovered: How Modern Science is Rewriting Their Story is the fact that we do not yet know
how the ancestors of Neanderthals first made it to Europe. In particular, we don’t know where
Homo antecessor fits in the Homo family tree. The book presents two hypotheses. The first
claims that the H. heidelbergensis were part of a second migration out of Africa, after the initial
Homo erectus migration, and TD6 were a smaller population of erectus-grade hominins that left
no descendants. The other hypothesis claims that H. antecessor went on to evolve into H.
heidelbergensis in Europe and later into Neanderthals, while archaic Homo like H. rhodesiensis
in Africa, despite looking similar, were on a completely different branch towards modern Homo
sapiens. Papagianni and Morse (2023) still present as live two possibilities: that H. antecessor
represents a failed early colonization or that it evolved locally into H. heidelbergensis. However,
the recent palaeoproteomic data published in 2020 makes the second one much less likely, and
other hypotheses are debated instead. I will be presenting evidence for or against these views.
When Homo antecessor was first described in 1997, Bermúdez de Castro and colleagues
explicitly proposed it as the best candidate for the last common ancestor of modern humans and
Neanderthals (Bermúdez de Castro et al, 1997). The TD6 material from Gran Dolina - mostly
fragmentary cranial bones, mandibles and teeth - was dated to roughly 0.9 million years ago and
interpreted as the earliest clear human presence in western Europe. On the basis of its
combination of primitive dentition (more similar to H. erectus) and a supposedly
“modern-looking” midface, the Atapuerca team argued that H. antecessor occupied a pivotal
position in the Homo tree, standing at the base of both the Neanderthal and H. sapiens lineages.
This interpretation, which effectively displaced H. heidelbergensis from the role of common
ancestor, strongly influenced how the species was discussed in the early 2000s and is the
backdrop for the two scenarios outlined by Papagianni and Morse.
In 2020, Welker et al published their analysis of tooth enamel proteins of Homo
antecessor. The study used several known proteins and compared amino acid substitutions that
arise with a species’s evolution, and the results consistently indicate that H. antecessor from TD6
was a close sister lineage to them, not an ancestral one. The strength of the proteomic evidence
lies in the fact that enamel proteins can survive for close to a million years, long after DNA has
degraded beyond recovery. Using maximum-likelihood and Bayesian phylogenetic methods on
the resulting amino-acid alignments, they consistently recovered H. antecessor as the closest
sister lineage to the clade containing H. sapiens, Neanderthals and Denisovans, with Dmanisi H.
erectus branching off deeper in the tree (Welker et al, 2020).
That makes it likely that neither the branching H. antecessor nor its descendants
contributed significantly to the modern human populations in Europe, so they most likely went
extinct before H. heidelbergensis re-colonized Europe. That appears to be the more widely
accepted version. The reason why this evidence doesn’t settle the debate is that while it more or
less proves that antecessor from TD6 itself isn’t ancestral to heidelbergensis, it doesn’t rule out
the possibility that an earlier antecessor-like form somewhere in Eurasia is. This way, the book’s
proposed idea that H. antecessor evolved into H. heidelbergensis in the strict sense is largely
disproven, but a softer version of it is still plausible, if unlikely. Additionally, making a claim
like “H. antecessor evolved into H. heidelbergensis” is inherently imprecise because the
definition of H. heidelbergensis and even its right for existence as a species is debated. Some
researchers use it very broadly, covering nearly all Middle Pleistocene Homo, including the
African specimens, others only apply it to the European variant, and some argue for the
retirement of the term (Roksandic, 2022). Some paleoanthropologists also claim that H.
antecessor’s morphological “modernity” compared to H. erectus can be explained by
developmental differences and do not warrant a separate species name. Freidline et al, however,
claim that while the humanlike features are not just developmental and would have persisted into
adulthood, they cannot be used to justify H. antecessor as ancestral to modern humans because
modern-like features appear in early homo often. Others, notably Welker et al, claim that the
“modern” face of H. antecessor might actually be an example of a more primitive morphology,
similar to that of the LCA of modern humans and Neanderthals, and the Neanderthals’
distinctive facial features are derived.
At the same time, the proteomic dataset has important limitations that leave some room
for alternative narratives. The enamel sequences come from a single tooth, representing a single
individual from one site. They also sample only a tiny portion of the genome, so they are
excellent for resolving broad branching order but much less informative about population
structure, gene flow and regional variation. The TD6 H. antecessor individual can therefore be
placed confidently as a sister to later Homo, but this does not mean that we have sampled every
early European population that might have contributed to the Neanderthal lineage. Given how
sparse the fossil record is between roughly 1.2 and 0.7 million years ago in Europe and
southwest Asia, it remains possible that other, as-yet-unknown antecessor-like groups in Eurasia
were closer to the true ancestral metapopulation, even if the specific TD6 population was not.
It’s important to note that this whole discussion depends on a single fossil site - TD6 in
the Gran Dolina cave at Atapuerca, northern Spain. It remains the only fossil site of H.
antecessor, with about 11 individuals, from roughly 0.9 million years ago. This means that any
proposed role and position in the evolutionary tree for the species is heavily restricted both in
space and time.
References
Welker, Frido, Jazmín Ramos-Madrigal, Petra Gutenbrunner, Meaghan Mackie, Shivani
Tiwary, Rosa Rakownikow Jersie-Christensen, Cristina Chiva, et al. 2020. “The Dental Proteome
of Homo Antecessor.” Nature (London) 580 (7802). London: Nature Publishing Group UK:
235–38. doi:10.1038/s41586-020-2153-8.
J. M. Bermúdez de Castro, J. L Arsuaga, E Carbonell, A Rosas, I Martínez, and M
Mosquera. 1997. “A Hominid from the Lower Pleistocene of Atapuerca, Spain: Possible
Ancestor to Neandertals and Modern Humans.” Science (American Association for the
Advancement of Science) 276 (5317). Washington, DC: American Society for the Advancement
of Science: 1392–95. doi:10.1126/science.276.5317.1392.
Roksandic, Mirjana, Predrag Radović, Xiu‐Jie Wu, and Christopher J Bae. 2022.
“Resolving the ‘Muddle in the Middle’: The Case for Homo Bodoensis Sp. Nov.” Evolutionary
Anthropology 31 (1). Hoboken, USA: John Wiley & Sons, Inc: 20–29. doi:10.1002/evan.21929.
Freidline, Sarah E, Philipp Gunz, Katerina Harvati, and Jean-Jacques Hublin. 2013.
“Evaluating Developmental Shape Changes in Homo Antecessor Subadult Facial Morphology.”
Journal of Human Evolution 65 (4). Kidlington: Elsevier Ltd: 404–23.
doi:10.1016/j.jhevol.2013.07.012.
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