Speech is not impossible with an ape vocal tract, but merely less expressive, with fewer vowels available. Furthermore, the vocal tract in living mammals is quite flexible, and a resting position different from the human configuration does not preclude a dynamically lowered larynx, giving near-human vocal capabilities, during vocalizations.
Adaptations for speech can be found in our speech organs, hearing organs, the neural connections between these organs, as well as the genes controlling their development.
- Speech organs. The shape of the human vocal tract, notably the permanently lowered larynx is very likely a speech adaptation, even though some other mammals, such as big cats, also possess a lowered larynx. The vocal tract itself is all soft tissue and does not fossilize, but its shape is connected with the shape of the surrounding bones, the skull base and the hyoid. Already Homo erectus had a near-modern skull base, but the significance of this is unclear, and other factors than vocal tract configuration, notably brain size and face size, strongly affect skull base shape. Hyoid bones are very rare as fossils, as they are not attached to the rest of the skeleton, but one Neanderthal hyoid has been found, as well as two hyoids from Homo heidelbergensis, all very similar to the hyoid of modern Homo sapiens, leading to the conclusion that Neanderthals had a vocal tract adequate for speech. The hyoid of Australopithecus afarensis, on the other hand, is more chimpanzee-like in its morphology, and the vocal tract that reconstruct for Australopithecus is basically apelike.
- Hearing organs. Some fine-tuning appears to have taken place during human evolution to optimize speech perception, notably our improved perception of sounds in the 2-4 kHz range. The sensitivity of ape ears has a minimum in this range, but human ears do not, mainly due to minor changes in the ear ossicles, the tiny bones that conduct sound from the eardrum to the inner ear. This difference is very likely an adaptation to speech perception, as key features of some speech sounds are in this region. The adaptation interpretation is strengthened by the discovery that a middle-ear structural gene has been the subject of strong natural selection in the human lineage These changes in the ossicles were present already in the 400,000-year-old fossils from Spain, well before the advent of modern Homo sapiens. These fossils are most likely Homo heidelbergensis. In the Middle East, ear ossicles have been found both from Neanderthals and from early Homo Sapiens, likewise with no meaningful differences from modern humans.
- Lateralization. There is no clearcut increase in general lateralization of the brain in human evolution — ape brains are not symmetric — and fossils are rarely undamaged and undistorted enough to be informative in this respect. But when tools become common, handedness can be inferred from asymmetries in the knapping process, the usewear damage on tools, and also in tooth wear patterns, which may provide circumstantial evidence of lateralization, and possibly language. Among apes there may be marginally significant handedness, but nothing like the strong population-level dominance of right-handers that we find in all human populations. Evidence for a human handedness pattern is clear among Neanderthals and their predecessors in Europe, as far back as 500 kya, and some indications go back as far as 1 mya. To what extent conclusions can be drawn from handedness to lateralization for linguistic purposes is, however, unclear.
- Neural connections. Where nerves pass through bone, a hole is left that can be seen in well-preserved fossils. Such nerve canals provide a rough estimate of the size of the nerve that passed through them. A thicker nerve means more neurons, and presumably improved sensitivity and control. The hypoglossal canal, leading to the tongue, has been invoked in this context, but broader comparative samples have shown that it is not useful as an indicator of speech. A better case can be made for the nerves to the thorax, presumably for breathing control. Both modern humans and Neanderthals have wide canals here, whereas Homo erectus has the narrow canals typical of other apes, indicating that the canals expanded somewhere between 0.5 and 1.5 million years ago.
- FOXP2. When mutations in the gene FOXP2 were associated with specific language impairment, and it was shown that the gene had changed along the human lineage, it was heralded as a “language gene”. But intensive research has revealed a more complex story, with FOXP2 controlling synaptic plasticity in the basal ganglia rather than language per se, and playing a role in vocalizations and vocal learning in a wide variety of species, from bats to songbirds. Nevertheless, the changes in FOXP2 in the human lineage quite likely are connected with some aspect of language, even if the connection is not as direct as early reports claimed. Relevant for the timing of the emergence of human language is that the derived human form of FOXP2 was shared with Neanderthals, and that the selective sweep driving that form to fixation may have taken place more than a million years ago, well before the split between Homo Sapiens and Neanderthals.
No single one of these indications is compelling on its own, but their consilience strengthens the case for some form of speech adaptations in Homo Heidelbergensis.
As the speech optimization, with its accompanying costs, would not occur without strong selective pressure for complex vocalizations, presumably verbal communication, this implies that Homo erectus already possessed non-trivial language abilities. While Homo erectus did not possess our species’ ability for ratcheting (cumulative) culture, it did exhibit art and sufficient skills to construct watercraft.