Evolution TL: March to Life > Evolution > Great Apes > Human > Consciousness > All to Us
(Plants | Animals > Cephlapods | Insects | Fish > Amphibians > Reptiles > Dinosaurs & Birds | Synapsids & Mammals)
Both reptiles and our ancestor synapsids evolved from amphibians. While reptiles evolved better amniotic eggs, synapsid eggs were like amphibian eggs. Synapsid’s birthing process eventually led to mammalian live births. These are the animals that evolved Complex Sentience, the ability to feel various emotions. While it is unknown when this complex spectrum fully evolved, it is defined as the ability to suffer and feel the dichotomy of pleasure and pain. Dimetrodon is an example of a meat eater, which if any of our ancestors were meat eaters. Although not a While dimetrodons were not direct ancestor of mammals, our mammalian ancestors might have been similar to our direct-line ancestors around this time.
Class: Synapsids (pre-mammal, not a dinosaur, not pre-dinosaur)
Evolution: Amphibians > Amniotes > Early Reptiles > Synapsids
Time Period: Late Triassic to Early Jurassic; 295-272 Mya; Diet: Carnivore
Around 240 million years ago, during the late Triassic period, a crucial evolutionary development unfolded within the lineage that would give rise to mammals. It is believed that the XX/XY sex-determination system emerged in a common ancestor of mammals, possibly within the genus Therapsida, a group of synapsids that exhibited both reptilian and mammalian traits. This innovation in genetic sex determination provided a reliable mechanism for differentiating male and female individuals based on the presence of the Y chromosome, which carries the SRY gene responsible for initiating male development. This advancement played a pivotal role in the diversification and success of early mammalian ancestors.
While the XX/XY system is prevalent in mammals, it is not exclusive to them, and it evolved independently in different animal groups. Some reptiles, such as certain species of lizards and snakes, as well as some fish, including the medaka (Japanese rice fish), also employ this genetic mechanism for sex determination. However, in mammals, this system is universal. From tiny rodents to massive elephants, all mammals utilize the XX/XY system, ensuring males carry one X and one Y chromosome (XY), while females carry two X chromosomes (XX). This consistency has allowed mammals to thrive in a variety of environments, contributing to their evolutionary success. Even in monotremes like the platypus, which exhibit a more complex array of sex chromosomes, the fundamental principles of the XX/XY system are still at play, highlighting the enduring legacy of this ancient genetic innovation.
References
- XX/XY Sex-Determination System in Mammals: Graves, J.A.M. (2006). “Sex Chromosome Specialization and Degeneration in Mammals.” Cell, 124(5), 901-914. doi:10.1016/j.cell.2006.02.023.
- Evolution of the XX/XY System: Bachtrog, D., Kirkpatrick, M., Mank, J.E., McDaniel, S.F., Pires, J.C., Rice, W., & Valenzuela, N. (2011). “Are all sex chromosomes created equal?” Trends in Genetics, 27(9), 350-357. doi:10.1016/j.tig.2011.05.005.
- Sex-Determination Systems in Reptiles and Fish: Ezaz, T., Sarre, S.D., & Georges, A. (2009). “Evolution of Sex Determination in Reptiles.” Sexual Development, 3(2-3), 109-117. doi:10.1159/000223073.
- Complex Sex Chromosomes in Monotremes: Grützner, F., Rens, W., Tsend-Ayush, E., El-Mogharbel, N., O’Brien, P.C.M., Jones, R.C., Ferguson-Smith, M.A., & Graves, J.A.M. (2004). “In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes.” Nature, 432, 913-917. doi:10.1038/nature03021
Morganucodon is an example of a plant eater likely similar to our direct-line ancestors around this time. It is not a direct human ancestor but is among the early mammaliaforms, close to the lineage leading to true mammals. It likely had a varied diet, but its inclusion here highlights the transition towards more specialized mammalian diets from the broader reptilian ones.
Evolution: Amphibians > Amniotes > Synapsids > Mammalia
Class: Mammaliaforms or maybe Synapsida
Time Period: Late Triassic to Early Jurassic
Diet: Likely Herbivore
Play in its most basic form likely began as simple, physical interactions aimed at honing survival skills. Modern animals like squirrels, kangaroos, and hedgehogs still exhibit these foundational behaviors, engaging in chasing, pouncing, and wrestling. These actions help young animals develop coordination and reflexes while providing a safe way to practice life-essential tasks. Surprisingly, even cows will play, especially young calves, who prance, jump, and head butt with an exuberance that reminds us how universal the joy of play can be.
An example of early live birth is the protomammal Kayentatherium, Jurassic period. This cynodont is related to early mammals and its clutch size suggested egg-laying, providing clues about the transition to live birth. The switch to live birth in mammals, including marsupials and placentals, likely evolved once at their common ancestor, suggesting live birth in mammals has a deep evolutionary history.
The rise of Eomaia scansoria, an early placental mammal, marks a definitive leap towards “Complex Sentience” in the evolutionary saga leading to humans. Unearthed from the Early Cretaceous period, Eomaia’s sophisticated array of mammalian features heralds the advent of deeply emotional and social behaviors. Possessing a brain and nervous system capable of supporting complex emotions, Eomaia represents a lineage increasingly equipped for the nuanced experiences of joy, suffering, pleasure, and pain. This small, shrew-like creature’s life history likely involved care for its young, suggesting the presence of emotional bonds and a capacity for emotional experiences that go beyond mere survival instincts. As one of the earliest examples in the mammalian lineage, Eomaia embodies the evolutionary moment when our ancestors began to navigate the world not just physically but emotionally, setting the stage for the rich tapestry of sentient experiences that characterize human life and our closest animal relatives today.
Likely Proto Self-aware: It’s plausible that it possessed a foundational level of self-awareness, or what can be termed as Proto Self-awareness. This rudimentary sense of self would not meet the criteria for self-recognition observed in modern species but likely included basic self-directed behaviors and an emerging sense of individuality advantageous for survival and social interactions. The cognitive capacities of its descendants suggest that the earliest forms of these traits, essential for navigating complex environments and social dynamics, began to develop around this pivotal point in mammalian evolution.
By about 125 million years ago, mammals likely increased their vocabulary above a dozen and perhaps for some species at times into the hundreds. This vocabulary, or signaling, was a series of gestures, grunts, and screeches. Today, mammals from this lineage communicate with a range of signals and gestures, not unlike our abstract vocabulary. We see this in their descendants ranging from the opossum, with communication signals of only about a dozen, to wolves, ranging into the hundreds.
Today, wolves use this rich tapestry of communication in the wild: howls to signal location, growls to assert dominance, and a myriad of body postures to convey submission, aggression, or affection. This early foundation of dozens of signals provided the building blocks for the more complex communication systems that would evolve in other branches of the mammalian family tree.
Imagined Image: Eomaia scansoria in their natural environment from about 125 million years ago. These early mammals likely lived in a lush, prehistoric forest setting and had a vocabulary, or signaling, ranging into perhaps a dozen or two of words—well, gestures.
The appendix is a small, finger-shaped pouch attached to the large intestine. It has long been considered a vestigial organ, meaning that it has no function in the human body. However, recent research suggests that the appendix may actually serve as a reservoir for beneficial gut bacteria.
The appendix is an example of a Phenotype Variation — a trait that varies among individuals. In fact, something like 1 in 100,000 people are born without an appendix. The presence or absence of the appendix is one example of a variation in humans. However, the presence or absence of the appendix is not a typical example of phenotype variation, as it is not a continuous range of variation within a population. Nonetheless, it is an interesting variation that our descendants over the next eons will certainly observe.
In more advanced forms, play becomes a complex tool for social bonding, emotional learning, and cognitive development. Carnivores like wolves and cats engage in cooperative games that refine group hunting skills, while elephants are known for their intricate, playful interactions that build emotional connections. Among primates, play takes on its most elaborate forms, fostering problem-solving, social hierarchies, and even creative role-playing, demonstrating the profound depth of higher play in nature.
Within mammals, only primates have binocular vision, grasping hands, and flat nails–instead of claws. Purgatorius might have had all three earning it the earliest known proto-primate label. It lived in Eastern Montana about 66 million years ago during the very last years of the Cretaceous period. It lived through the K-T extinction event and the extinction of dinosaurs.
Class: Mammal; Early Proto-Primate
Time Period: Late Cretaceous to the early Paleocene
Diet: Likely Frugivorous (fruits) / Insectivorous (insects)
Around 60 million years ago, the early ancestors of primates began to develop a crucial adaptation: the opposable thumb. This evolutionary milestone marked the beginning of increased dexterity and the ability to grasp objects more effectively. Living in the dense canopies of prehistoric forests, these early primates used their newly opposable thumbs to navigate their environment, forage for food, and interact with each other in more complex ways. This small but significant change laid the groundwork for the remarkable manual dexterity that would evolve in future primate species.
Plesiadapis, a proto-primate, is an example of a fruit-insect eater likely similar to our direct-line ancestors around this time.
Class: Mammal; Early Primate (Plesiadapiformes)
Time Period: Late Paleocene
Diet: Likely Frugivorous/Insectivorous
Emerging in the lush forests of the Eocene, Miacis signifies a pivotal moment in the evolution of cognitive abilities among mammals. As a basal member of the Carnivora, this small, tree-dwelling creature exhibited behaviors and social dynamics suggesting the early stages of self-awareness. Though not akin to the self-recognition seen in humans or other highly intelligent animals, the life of Miacis and its interactions with its environment and conspecifics likely involved a level of awareness and individual recognition. These early forms of cognitive complexity mark the beginning of the path toward the rich inner lives characterized by self-awareness in later mammals.
Likely between 25 and 30 million years ago, the evolution of the apes brought about further refinement of the opposable thumb. This period saw the divergence of the lineages that would lead to modern “lesser” apes like gibbons as well as later “great” apes, including orangutans, gorillas, and chimpanzees. The ape thumb evolved to become more robust and versatile, allowing these primates to perform a wider range of tasks. The ability to grasp and manipulate objects with precision became increasingly important for activities such as tool use, social interactions, and foraging. This evolutionary step set the stage for the remarkable capabilities seen in modern apes and, eventually, in humans.
Image: Gorilla on right, human, then orangutan. Orangutan-like hands evolved about 30 mya, gorilla-like hands evolved about 12 mya, and human-like hands evolved about 3 mya.
True Primate: Within mammals, only primates have binocular vision, grasping hands, and flat nails–instead of claws.
Intelligent: Within the dense forests of the Oligocene epoch, Aegyptopithecus zeuxis marked a significant advance in the evolution of intelligence among primates. As an early forerunner to both the great apes and humans, Aegyptopithecus possessed adaptations crucial for enhanced cognitive function, such as a larger brain relative to its body size and eyes positioned for depth perception. These physical traits supported the development of behaviors requiring problem-solving, learning, and adaptation—hallmarks of emerging intelligence. The social life of Aegyptopithecus, inferred from its anatomy and fossil context, likely involved complex interactions and the use of rudimentary tools, setting the stage for the exponential growth in intelligence that characterizes later primates, including humans.
Around 25 to 29 million years ago, Old World monkeys, including species like baboons and macaques, branched off from the common ancestor shared with apes. Unlike their ape cousins, Old World monkeys retained their tails and adapted to a wider range of habitats, from forests to savannas. Baboons, known for their ground-dwelling behavior, exhibit strong social structures and advanced communication. However, they lack the opposable thumbs and tool use that evolved in the ape lineage. This branching marked a key evolutionary moment, setting the stage for the development of the tailless apes, including gibbons and later great apes.
Great Apes LCA candidate: Proconsul, an inhabitant of the Miocene forests in East Africa, stands as a landmark in the evolutionary journey toward self-awareness. This early ape lacked a tail and exhibited a mixture of arboreal and terrestrial traits, providing clues to the social and environmental challenges that likely spurred cognitive advancements. While direct evidence of self-awareness in Proconsul is beyond our reach, its position in the ape lineage suggests the development of social structures and cognitive abilities that predate the sophisticated self-awareness observed in modern great apes, elephants, and dolphins. Proconsul’s world was one of increasing cognitive complexity, setting the stage for the emergence of true self-aware beings, capable of recognizing themselves as distinct entities within their social and natural environments.
The last surviving primate before the great apes! Gibbons are a “lesser ape” that branched off from our common ancestor with great apes. By this time, our ancestors had already lost their tails (like all apes) and were very agile in trees, using brachiation (arm-swinging) to move through the canopy. Gibbons are known for their intricate vocalizations, particularly the duets they sing to communicate with their mates and establish territory. Bipedalism in our ancestors has roots around this time, as gibbons evolved an upright posture while moving through the trees and on the ground, where they walk bipedally for short distances. Around this period, our ancestors also began to rely more on vision than smell, which many researchers believe played a significant role in the development of larger brains. This shift required more complex brain structures to process the increasingly sophisticated visual information needed for survival.
Gibbons do not exhibit the same advanced tool use or cognitive complexity seen in great apes like orangutans, which branched off a few million years later. Gibbons are not as adept at problem-solving, have weaker memory, and show fewer social skills. For example, they do not laugh—a behavior linked to emotional intelligence. Additionally, while orangutans use and even create tools, gibbons do not.
Orangutan ancestor: After the Great Apes LCA, orangutans evolved in Asia. The genus Sivapithecus represents early orangutans. An extinct species of the great apes, they lived in the Indian subcontinent from around 12 to about 8 million years ago. It is considered a close relative of the orangutan lineage and shares many similarities with modern orangutans, including a similar skull shape and dental structure. Sivapithecus indicus had a more advanced brain than earlier great apes, and its face was likely more protruding and snout-like, similar to modern orangutans. Its discovery in the Siwalik Hills of India and Pakistan has provided important insights into the evolution of the great ape lineage in Asia, and it is thought to have played a key role in the origins of the orangutan genus, Pongo.
Last Gorilla-Chimp-Human ancestor: The last common ancestor of gorillas, chimpanzees, and humans likely lived about 8 to 10 million years ago. Both the Nakalipithecus and Chororapithecus genuses are candidates. Genus Nakalipithecus: This “might” be the leading plausible candidate. From Kenya, they are dated to about 10 million years ago.
Location: Emerged in Western or Central Africa; spread to Eastern Africa and no further.
Size: Up to 5’8″ and 550 pounds (Eastern Lowland Gorillas)
Around 1.5 to 2 million years ago, the evolutionary branches of ancient primates led to the distinct emergence of what we now recognize as the chimpanzee family, under the genus “Pan.” This pivotal event in primate evolution unfolded approximately 5 million years after our last common ancestor with chimpanzees and bonobos took separate paths. As with many significant chapters in the story of primates, this one too unfolded on the diverse and vibrant stage of Africa, a continent that has been the cradle for the unfolding drama of human and primate evolution alike. This era marks not just the divergence of chimpanzees and bonobos but a defining moment in the rich tapestry of hominid history.
Chimps are smart, but nowhere near Homo habilis from 2.3 million years ago.
Size: 3’ to 4’5″ (a bit shorter than Homo habilis)
Brain Size: 273 to 500 cm³ (much smaller than Homo habilis at 510 to 600 cm³)
Brain to Body EQ: 2.2 to 2.5 (much less than Homo habilis at 3.3 to 3.8, and humans at 7.4 to 7.8)
Orangutans (genus Pongo) are great apes native to Indonesia and Malaysia, characterized by their distinctive reddish-brown fur, long arms, and intelligent behavior. They are the most arboreal of all great apes, spending most of their time in trees, and are skilled climbers and swing between branches with ease. Orangutans are also known for their advanced problem-solving abilities and have been observed using tools, such as sticks, to extract food and navigate their environment. With their slow pace of life and solitary nature, orangutans have adapted to their forest habitat in unique ways, making them one of the most fascinating and endangered primate species.
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