Co-evolution theory suggests that the evolution of language is closely intertwined with cultural innovation and learning. It emphasizes the gradual development of language skills, such as recursion, within the contexts of toolmaking, social collaboration, and communication. In other words, language evolves in conjunction with cultural advancements and social practices, shaping and being shaped by human interactions and innovations.

The gestural theory suggests that human language originated from gestures used for basic communication. According to this theory, early humans communicated through gestures before developing vocal language. Evidence supporting this idea includes similarities in neural systems for gestural and vocal language, as well as the proximity of brain regions responsible for mouth and hand movements in the cortex.

The Himalayas theory posits that the development and evolution of natural language are influenced by geographical landforms such as mountains, rivers, oceans, as well as climatic conditions and environments at different locations around the Earth. The Himalayas mountain ranges serve as a significant natural barrier for human migration and language evolution. According to this theory in linguistics, human languages globally are divided into two major categories: Phonographic languages and Ideographic languages, separated by the Himalayas and its neighboring mountain ranges. The West Himalayas languages (WHL) are predominantly phonographic, while some East Himalayas languages (EHL), like Chinese, are ideographic. Languages in the northern and southern regions of the Himalayas are influenced by both WHL and EHL, resulting in “Mixed Himalayas languages” (MHL). Research based on the “International Alphabet Text” (IPT) database has identified over 100 languages as phonetic or ideographic from the West Himalayas and East Himalayas, as well as the surrounding areas. However, further research employing diverse linguistic approaches and methods is needed to validate the Himalayas Theory. (More citations needed)

Interlanguage theory in linguistics refers to the transitional linguistic system that learners create as they progress toward acquiring a second language. It reflects a learner’s evolving understanding of the language, incorporating elements from their native language and the target language. Interlanguage is dynamic and changes over time as learners gain proficiency.

Mirror system theory, also known as the Mirror Neuron System (MNS) or Mirror System Hypothesis, suggests that certain brain cells, called mirror neurons, enable individuals to imitate the actions and behaviors of others. These mirror neurons fire both when an individual performs a specific action and when they observe someone else performing the same action. This neural mirroring mechanism is believed to play a crucial role in social learning, empathy, and understanding the intentions and emotions of others. In the context of language development, mirror system theory proposes that mirror neurons contribute to our ability to imitate and learn language, allowing us to understand and replicate the complex motor skills and gestures involved in communication.

Monogenesis theory, also known as the Out of Africa theory or the Recent African Origin theory, posits that all modern humans share a common ancestry that originated in Africa. According to this theory, anatomically modern humans first appeared in Africa and then migrated and dispersed to other parts of the world, eventually replacing other hominid populations, such as Neanderthals and Denisovans. Genetic and fossil evidence support the idea that modern humans originated in Africa and gradually populated other regions, leading to the diversity of human populations we see today.

Mother-Child Interaction Theory, also known as parent-child interaction theory, emphasizes the significant impact parents, particularly mothers, have on their child’s development. It posits that a child’s ability to regulate their emotions and develop social, language, and cognitive skills is strongly influenced by the quality of interactions with their parents, especially during the early years of life. This theory highlights the importance of parental responsiveness to an infant’s emotional cues and needs, which fosters a secure attachment and promotes healthy development in various areas of the child’s life. Positive and nurturing interactions between mothers (or parents) and children are believed to be crucial for the child’s overall well-being and successful socialization.

Polygenesis theory, in the context of human evolution and linguistics, posits that different human populations or groups evolved independently in separate geographic locations. This theory suggests that various groups of early humans evolved into distinct species in different parts of the world, rather than originating from a single common ancestor. In the field of linguistics, polygenesis specifically refers to the idea that human languages evolved separately in multiple regions, resulting in different language lineages that are not necessarily interconnected.

Polygenesis theory suggests that hominids migrated out of Africa at different times and that early human ancestors may have evolved in places outside of Africa. Linguistically, it argues that language use originated from multiple locations, leading to the wide variety of languages and language families we observe today. Polygenesis theory contrasts with monogenesis, which proposes that all human languages trace back to a single common ancestor.

Universal Grammar is a theory in linguistics that suggests that the ability to acquire language is innate to humans, and that humans are born with an inherent knowledge of the structure and rules of language. Proposed by Noam Chomsky, this theory suggests that all languages share a common underlying structure, and the differences between languages are variations within this universal grammar. According to this theory, humans are biologically predisposed to learn languages and have an inherent understanding of grammar, which allows them to acquire any language they are exposed to during their early years. Universal Grammar theory has had a significant influence on the study of linguistics and language acquisition.

Vocalization theory refers to the study and understanding of vocalizations in various organisms, including humans and animals. It explores the mechanisms, functions, and evolution of vocal communication. Researchers in this field analyze the sounds produced by different species, investigating the purposes of these vocalizations, such as mating calls, warning signals, or social interactions. By studying vocalization patterns, scientists can gain insights into the behavior and communication systems of different species.