The following statements include several guesses
because no one has a talent for reading the ‘mind’ of a flatworm or any other animal. Levels of consciousness are surmised and estimated from sensory organs present and recorded behavior.
Bear in mind
that at the genesis of flatworms and roundworms there were no more advanced or dangerous predators present.
However, thereafter and until the present day,
a series of both vertebrate and invertebrate predators became important factors.
1 Flatworms (basal bilaterals ~ Ediacaran)
have a need and ability to get from ‘here’ to ‘there’, driven by desire (e.g. hunger) and thereafter finding shelter after satisfied. Flatworms have a ‘front’ in which primitive eyes capable of seeing blurry light vs dark are present. Nerve tissues are also concentrated toward the front. The ‘mouth’ is also the ‘anus’ and this opening is located ventrally and centrally in primitive flatworms. Flatworms sense pain (measured by a reflex response) of various sorts (physical, thermal, chemical). Flatworms seek the chemtrails of potential mates and avoid those of predators.
2 Ribbonworms and Roundworms (Cambrian)
The nervous system is concentrated in a single, central nerve chord and the skin is tough, but sprinkled with nerve endings. The mouth has migrated to the front while the anus has migrated to the rear. Roundworms dig to avoid predators and seek food using whole body all-axis undulation, rather than the cilia found beneath primitive flatworms. Fertilization is internal with genders separated into male and female, so not all conspecific individuals are potential mates. Pheromones are likely produced (and sensed) to indicate gender with sexual excitement present prior to fertilization.
Figure 1. Extant lancelet (genus: Amphioxus) in cross section and lateral view. The gill basket nearly fills an atrium, which intakes water + food, sends the food into the intestine and expels the rest of the water.
3 Lancelets (basal chordates ~ Cambrian)
Coordinated swimming (restricted to lateral undulations), in a 3D environment. Lancelets collect in colonies, half-buried in sand. Drifting plankton are snared by mucous strands inside the oral cavity. Males expel sperm and females expel eggs for external fertilization on a mutual chemical stimulus (taste + olfaction) released into water.
4 Fish (basal vertebrates, craniates ~ Ordovician, Silurian)
Reversing back to flat and round worms (in contrast to living lancelets), basal fish pursue their prey which, consist of tiny defenseless organisms primitively (e.g. whale sharks, manta rays), building slowly to larger prey. More sensitive sensory organs (eyes with a lens and cornea + nostrils) and a brain appear, along with balancing organs (semi-circular canals) and lateral lines, increasing the sensory input. The tail pushes the head where it wants to go (e.g. toward prey), like a rocket engine on a guided missile. Sharks see in shades of gray. Bony fish see in color. Some fish migrate during ontogeny (from spawning, to feeding and nursery zones). Thus migration is driven by a subconscious (instinctual) sense of time.
Figure 2. Loganellia, from the Silurian, had a large, wide gill chamber inside a low skull.
5 Amphibians (basal tetrapods ~ Devonian)
At their genesis amphibians clamber through muddy waters, then soft and very wet muddy and sandy banks, then more substantial substrates above the surface of the water. Amphibians are instinctively driven back to water for sex and egg-laying. As in primitive fish, hundreds of eggs are laid and no care is offered to hatchlings of basal amphibians. Terrestrial amphibians can get thirsty, so they absorb water through the permeable skin of their belly and inner thighs when needed. Frogs have eardrums for hearing social frog songs and croaks. Amphibans have a nictating membrane and tear glands to moisten the eyeball. A fleshy tongue is not only more sensitive to irritants and poisons, but muscular and capable of manipulating food. Amphibians retreat underwater when disturbance is detected by others nearby. Amphibians lose sensory organ sensitive to aquatic vibrations: the lateral line system and porous bony scales,
6 Reptiles (basal amniotes – Early Carboniferous)
Typically silent, reptile mates are encountered by following scent trails. The nostrils are at the tip of the snout facilitating the detection of chemtrails. Internal fertilization requires cloaca to cloaca contact. Gravid females have greatly expanded abdomens prior to laying eggs. So they are heavy and awkward when the eggs are at their largest. With a slender stapes, reptiles are capable of hearing higher pitched sounds. Basal reptiles remain cold-blooded so they present themselves to warm sunlight and seek shadows when overheated.
Figure 3. Silvanerpeton from the Upper Viséan (331 mya) is the outgroup taxon for Gephyrostegus and the Amniota.
7 Basal synapsids (Early Permian)
Early in this lineage saber-toothed Biarmosuchus was an apex predator. In smaller, more derived taxa, families and colonies gathered in dens that are purposefully and instinctively dug anticipating a future need to avoid weather, enemies, etc. and to raise tiny young out of danger.
Figure 4. Biarmosuchus, a basal therapsid.
8 Basal mammals (Late Triassic)
Larger brains increase visualization (creating a mental map) during nocturnal activities in which hearing and olfaction came to dominate sight (with loss of color vision). Lactation and nursing increased hormonal releases that helped bond mothers to infants, which were tiny and helpless upon hatching.
9 Basal therians (Early Jurassic)
A prehensile ability was added to the tail (e.g. Didelphis, the opossum) as a sensory and locomotory organ. This was lost in many derived marsupials. Young were no longer raised in a den, but were carried about by the mother until large enough to survive on their own. This time period permitted learning for the young and extended the sense of a mother’s love and responsibility for her juveniles.
Figure 5. The marsupial, Monodelphis domestica, nests as a sister to Eomaia, the oldest known placental.
10 Basal arboreal placentals (Middle Jurassic)
Increased metabolism and brain size led to faster locomotion in a 3D arboreal environment that included leaping, often with young attached. Nests were built for some young to permit the mother to roam seeking more food. This was required (3x their own weight every day) by faster growing juveniles. Rapid-eye-movement dreaming first appears. Basal placentals can be aggressive to their own kind. Patrol areas are marked off with scent glands and body waste.
Figure 2. Human evolution back to the cynodonts, some 230 mya.
11 Basal primates (Cretaceous)
Increasingly arboreal, primates have better hand-eye coordination. Objects are brought to the nose and mouth by the hands. The hallux and pollex (thumb and medial toe) become better at grasping and manipulating objects. Social vocalization reappears (from the days of amphibians). Colonies appear, some with leaders and followers, facilitated by dominance and submission. Aggression and mutual grooming may keep the group in order. Individuals are identified by variations in the face and hair. Facial expressions originate here, at first by drawing back the lips to expose the canines. Binocular vision appears. Color vision and dreaming returns with a switch to a diurnal (daylight) existence. Rather than nervously cocking their heads to estimate leaping distance, primates maintain a steady gaze, like that of a cat. Nictating membranes disappear. Primates have a longer pregnancy with fewer young, down to one at a time, rarely twins. Vegetation added to the diet. The lifespan becomes longer (e.g. 18 years for lemurs) facilitating multi-generational help in raising young.
12 Basal anthropoids (apes ~ Eocene)
Delight, humor and sympathy appear. A sense of self is apparent. Sitting on haunches takes more time. The loss of the tail includes loss of the use of this appendage for sensory, signaling and locomotory functions. In its place a new form of locomotion: brachiation (hanging and swinging from one branch to another). Apes are more adaptable and teachable. They hide food for later use. Sexual union is used for more than just reproduction (e.g. dispute settlement).
13 Humans (recent)
This is when the behavioral growth curve of consciousness ascends exponentially. With increased brain size and larger social groups comes object trading, cooperation and coordination with strangers, music, art, dance, speech, invention and respect for the dead (e.g. ceremonial burial). Bipedal locomotion frees the hands to carry things (e.g. possessions, children, food and weapons). Pair-bonding replaces harem collecting due to greater interdependence on tribe members. Tribes are formed for mutual defense and offense. Walking and sharing food evolves into nomadic tribal culture. The ability to throw rocks and spears enables humans to kill without the risk of hand-to-hand, tooth and claw killing.
Instinct reduced, learning enhanced
The brain increases in size as cooperation during the hunt, muscular coordination for the hurling of stones, and the mental ability to detect deception become more and more important. The nose opens ventrally, permitting dives underwater.
Visual signals for sexual receptivity
become hidden when females assume a bipedal configuration, so the frenzy of a mating season is reduced. A courtship period (e.g. dating) aids to establish mutual trust. The rate of reproduction rises to about 5x that of apes due to being able to carry infants in various ways and by the aid given by neighbors and older tribe members.
are the brain’s way of ‘keeping the motor running’ while asleep, helping to store memories. Dreams work in the subconscious with images generated without sensory input, often with strangers, strange places and strange goals that go beyond an individual’s ability to decode them.
(e.g. parrots, dogs, elephants, dolphins) had their own evolution of consciousness at appropriate nodes after shared ancestors. Aside from humans, there are only a handful of animals able to recognize themselves in mirrors: elephants, gorillas, Rhesus monkeys, magpies, dolphins, orcas, pigs.
Some YouTube videos
(below) may prove helpful for additional information on this topic.
Peters D 1991. From the beginning – the story of human evolution. Little Brown. PDF