Table 3.
Timeline of the emergence of critical features of sensory consciousness in vertebrates.
| Feature/Taxon | Timeframe1 (million years ago) | Earliest fossil evidence (animal fossils) | References |
|---|---|---|---|
| Ediacaran period | 635–541 | Sponges | Erwin et al., 2011; Erwin and Valentine, 2013 |
| Cambrian period | 541–488 | Worm trace: Treptichnus pedum | Peterson et al., 2008; Erwin and Valentine, 2013 |
| 1. First chordate | 560–520 | No direct fossil evidence | See Figure 1, based on earliest body fossils of any bilaterian animals being about 560 my old (Erwin and Valentine, 2013) |
| 2. Common ancestor of tunicates and vertebrates (and first precursors of placodes?) | 560–520 | Shankouclava (fossil tunicate) | See Figure 1; Chen et al., 2003; Sestak et al., 2013 |
| 3. Paired lateral eyes | 560–520 | Haikouichthys | Shu et al., 1999, 2003, 2009; Butler, 2000; Vopalensky et al., 2012 |
| 4. Fully differentiated tri-partite brain2 | 560–520 (460) | Haikouella, or the first gnathostome-fish fossils2 | Janvier, 1996; Butler, 2000, 2006; Mallatt and Chen, 2003; Murakami and Kuratani, 2008; Sprecher, 2009 |
| 4. Cephalate animal | 560–520 | Hypothetical, so no fossil evidence | Butler, 2000, 2006 |
| 5. Placodes and neural crest | 560–520 | Haikouella and Haikouichthyes | Shu et al., 1999, 2003, 2009; Mallatt and Chen, 2003; Hall and Gillis, 2013 |
| 5. Isomorphic neural representations2 | 560–520 (460) | Murakami and Kuratani, 2008; Stephenson-Jones, 2012 | |
| 5. Non-visual sense organs-1 (olfaction, trigeminal somatosensory) | 560–520 | Haikouichthys (and Haikouella?) | Mallatt and Chen, 2003; Shu, 2003; Shu et al., 2003 |
| 5. Sister group of vertebrates: Haikouella | 520 | Yunnanozoans | Chen et al., 1999; Chen, 2012 |
| 6. Non-visual sense organs-2 (equilibrium, taste? lateral line?)2 | 560–520 (460) | Haikouichthys, Astraspis, and Sacabambaspis | Sansom et al., 1997; Braun and Northcutt, 1998; Shu, 2003; Shu et al., 2003 |
| 7. Vertebrate: Haikouichthys | 520 | Haikouichthys and related genera | Shu et al., 1999, 2003, 2009 |
Note the near-simultaneity of appearance of all features, near the time of vertebrate origin.
Features and taxa are numbered as 1–7 in the estimated order of their evolutionary appearance. Repeated numbers mean the different events occurred together or nearly simultaneously.
Dates of the features are taken from the fossil record (see text).
These features are not directly observable in the 520 million-year-old Haikouichthys fossils, but are inferred to have existed in that vertebrate because some correlated structures did. Conceivably, the feature might date to as late as the cyclostome (agnathan)-gnathostome split at 460 mya (Mallatt, 1996), but no later because both lampreys and gnathostomes have it.