Coombs and Trestman propose that species from three out of 17 phyla are much more advanced than those from the other 14. Their approach sounds like a revival of scala naturae attempts from more than half a century ago (e.g., Hodos & Campbell, Reference Hodos and Campbell1969), but with a wider perspective. Aside from the fact that already one of these three phyla (euarthropoda) makes up three quarters of all known living species (Meusemann et al., Reference Meusemann, von Reumont, Simon, Roeding, Strauss, Kück, Ebersberger, Walzl, Pass, Breuers, Achter, von Haeseler, Burmester, Hadrys, Wägele and Misof2010), there is (a) a wide variety of species with a very limited versus a huge repertoire of cognitive traits and neural/brain ramifications, possessing the identified “pivotal features” (flexible limbs, high resolution eyes, laminated brain structures, high-speed locomotion, eye mobility and 3D rotational senses), and (b) a wide variety of behavioral as well as neural traits that can be listed as being “advantageous” or key characteristics within each of the three phyla, which, however, differ immensely for different species of a given phylum (not to mention evolutionary selection pressures and ecological niches, acknowledged by the authors). The authors quite vaguely state for point (b) the existence of “3-D Motion Sense, UAL, CPF” as “behavior/cognitive traits … which include central place foraging (CPF) and unlimited associative learning (UAL) abilities” [their Fig. 3(A)].

The authors in our eyes center on more basic features – vision and motion – while for many neuroscientists, anthropologists, and philosophers, other attributes signal sophisticated or advanced evolutionary benefits – among them insightful behavior, fine-grained emotional reactivity, a developed self, and theory of mind functions. The central question therefore remains: Does it make sense to propose three phyla as outstanding from the others, when their members differ immensely with respect to cognition and nervous system and this both within and between the three phyla? Who would combine a spider, a lanternfish, an ape, and a pearly nautilus, especially, as the three top criteria – reserved for the “CC lineages only” (Fig. 3A) – certainly are barely applicable to a few of them (CC = cognitively complex)? The words of Howe and Courage (Reference Howe and Courage1993) “that there is not yet a sound empirical basis for making cross-species generalizations about the neural structures that mediate performance on the tasks used to assess memory” come to mind here and deserve consideration – especially with respect to learning and memory.

According to Shettleworth (Reference Shettleworth1993, p. 8), McGregor and Avery (Reference McGregor and Avery1986) wrote “that the conditions of learning differ [between species], as do their effects on behavior.” Shettleworth emphasized, as do we, that the kinds of learning and memory differ between species (e.g., imprinting and song learning in birds differ from imitation or concept learning in primates) and they involve very different brain regions even in animals from the same phylum.

“Associative learning” – as used by Coombs and Trestman – certainly is not a unity. It has to be divided into (1) short-term and long-term memory and (2) into several memory systems (Markowitsch & Staniloiu, Reference Markowitsch and Staniloiu2012, Reference Markowitsch and Staniloiu2022). Though the term long-term memory is used in euarthropod species (e.g., Collett & Collett, Reference Collett and Collett2002), long-term memory in bees and other insects certainly differs not only in duration from long-term memory in vertebrate animals living for many years. Those species with a “short” long-term memory period certainly lack the long-term memory capacity found in primates, whales, or elephants, where even evidence for intergenerational cultural traditions exists (e.g., washing of potatoes in Japanese macaques). Furthermore, since half a century, scientists have distinguished various memory systems (e.g., Mishkin & Petri, Reference Mishkin, Petri, Squire and Butters1984; Squire, Reference Squire2004; Tulving, Reference Tulving, Tulving and Donaldson1972, Reference Tulving, Terrace and Metcalfe2005) (Figure 1), which are now well established in vertebrates – with the exception of the hierarchically highest, the episodic-autobiographical memory system, which according to Tulving (Tulving, Reference Tulving, Terrace and Metcalfe2005; Tulving & Markowitsch, Reference Tulving and Markowitsch1998) may be reserved to (neurologically healthy) human beings. For other vertebrates, Clayton and others used the term episodic-like memory system (e.g., Davies & Clayton, Reference Davies and Clayton2024). Recently, Jozet-Alves, Schnell, and Clayton (Reference Jozet-Alves, Schnell and Clayton2023) looked for some memory comparabilities in cephalopods. Though finding only limited correspondencies, they concluded that studying cephalopods “might provide a unique opportunity to go further back in evolutionary time to pinpoint when certain cognitive abilities emerged” (p. R1095). Similar attempts apparently are missing for euarthropods, where researchers still use the general term memory (e.g., Dubnau et al., Reference Dubnau, Grady, Kitamoto and Tully2001; Menzel & Müller, Reference Menzel and Müller2001).

Figure 1.

The long-term memory systems (created together with Endel Tulving). Procedural memory represents motor-based skills, but also sensory and cognitive skills and routines. Priming refers to a higher likeliness of re-identifying previously perceived information. Perceptual memory allows distinguishing an item based on distinct features (e.g., an apple). Semantic memory is context-free and represents general facts (world knowledge). The episodic-autobiographical memory (EAM) system is context-specific with respect to time and place. It allows mental time travel and is based on self-reflection. Examples are events such as the last vacation. The five systems develop phylogenetically and ontogenetically from left to right. With respect to dimensions of consciousness, Tulving (Reference Tulving, Terrace and Metcalfe2005) considered the first two memory systems as being anoetic, the next two as noetic, and the EAM to be autonoetic (“self-conscious”). The lower part of the figure depicts the principal regions of the mammalian brain, which are engaged in the processing of each memory system.

But even if evidence for certain more complex forms of memory were found in species of the other phyla, such abilities would be far away from the refined and sophisticated compartmentalization, which exists in many mammalian species (Figure 1) and which serves them to maintain a lifelong repertoire of ideas and of flexible adaptations to rapidly changing environments.

And even if there were similar forms of memory between species of different phyla (or even within a phylum), this certainly does not imply that their neural bases would be similar (see, e.g., Bingham, Reference Bingham1992, and Figure 1 for brain structures involved in memory processing in mammals). But if they are not similar or homologous, they may imply different processes and different representations and features between species.