A Symbiotic View of Life: We Have Never Been Individuals Scott F. Gilbert, Jan Sapp, and Alfred I. Tauber argue that the traditional concept of the biological individual is flawed. Instead, organisms are better understood as holobionts—complex communities of host cells and persistent symbiotic microbes. This view challenges long-held notions of individuality in biology, emphasizing the role of symbiosis in evolution, development, and physiology. The authors highlight that animals are not isolated individuals but are composed of multiple species living in symbiotic relationships. For example, sponges host significant bacterial populations that contribute to host metabolism, and cows rely on gut symbionts for digestion. Similarly, corals depend on algal symbionts for nutrients, and the loss of these symbionts leads to coral bleaching. These examples show that symbiotic relationships are essential for the development, physiology, and survival of many organisms. Symbiosis also plays a critical role in genetic inheritance. Microbial symbionts can provide selectable genetic variation, influencing natural selection. The immune system also interacts with symbionts, developing in part through dialogue with these microbes. This integration of microbes into the host's biological systems challenges the traditional view of the immune system as a defense mechanism against external threats. The authors argue that the concept of the individual is not static but is shaped by the complex interactions within the holobiont. This perspective has implications for understanding evolution, development, and the immune system. It also challenges the classical view of individuality in biology, suggesting that organisms are better understood as integrated systems of multiple species. The paper also discusses the importance of symbiosis in various biological processes, including the development of organs, the completion of life cycles, and the regulation of physiological functions. For example, the development of the light organ in squid depends on symbiotic bacteria, and the immune system of germ-free mice is impaired without gut bacteria. The authors conclude that the traditional notion of individuality is outdated. Instead, organisms are better understood as holobionts, with their anatomical, physiological, and developmental functions shaped by symbiotic relationships. This new perspective has implications for medicine, conservation, and evolutionary biology, suggesting that the holobiont is a unit of natural selection. The paper emphasizes that the concept of individuality is not fixed but is a dynamic and context-dependent process.A Symbiotic View of Life: We Have Never Been Individuals Scott F. Gilbert, Jan Sapp, and Alfred I. Tauber argue that the traditional concept of the biological individual is flawed. Instead, organisms are better understood as holobionts—complex communities of host cells and persistent symbiotic microbes. This view challenges long-held notions of individuality in biology, emphasizing the role of symbiosis in evolution, development, and physiology. The authors highlight that animals are not isolated individuals but are composed of multiple species living in symbiotic relationships. For example, sponges host significant bacterial populations that contribute to host metabolism, and cows rely on gut symbionts for digestion. Similarly, corals depend on algal symbionts for nutrients, and the loss of these symbionts leads to coral bleaching. These examples show that symbiotic relationships are essential for the development, physiology, and survival of many organisms. Symbiosis also plays a critical role in genetic inheritance. Microbial symbionts can provide selectable genetic variation, influencing natural selection. The immune system also interacts with symbionts, developing in part through dialogue with these microbes. This integration of microbes into the host's biological systems challenges the traditional view of the immune system as a defense mechanism against external threats. The authors argue that the concept of the individual is not static but is shaped by the complex interactions within the holobiont. This perspective has implications for understanding evolution, development, and the immune system. It also challenges the classical view of individuality in biology, suggesting that organisms are better understood as integrated systems of multiple species. The paper also discusses the importance of symbiosis in various biological processes, including the development of organs, the completion of life cycles, and the regulation of physiological functions. For example, the development of the light organ in squid depends on symbiotic bacteria, and the immune system of germ-free mice is impaired without gut bacteria. The authors conclude that the traditional notion of individuality is outdated. Instead, organisms are better understood as holobionts, with their anatomical, physiological, and developmental functions shaped by symbiotic relationships. This new perspective has implications for medicine, conservation, and evolutionary biology, suggesting that the holobiont is a unit of natural selection. The paper emphasizes that the concept of individuality is not fixed but is a dynamic and context-dependent process.