The matter composing a newborn baby is not newly created but represents a temporary collection of atoms that have existed since the dawn of the universe. Every particle within the infant’s body, from the hydrogen in its water molecules to the iron in its blood, has an ancient history stretching back billions of years. This assembly involves a journey spanning cosmic events, geological cycles, and the intricate biological machinery of a parent. Tracing the origin of these atoms reveals a connection between the smallest unit of life and the largest structures in the cosmos.
The Cosmic Forge
The journey of the atoms that form a human life begins in space. The lightest elements, primarily hydrogen and helium, originated just moments after the Big Bang approximately 13.8 billion years ago. Hydrogen atoms, which constitute roughly 60% of the atoms in the human body, were formed during this initial rapid expansion and cooling phase.
Heavier elements necessary for life, such as carbon, oxygen, and nitrogen, were forged later inside the cores of stars through stellar nucleosynthesis. Small stars, like our Sun, fuse hydrogen into helium, and then, during the red giant phase, they fuse helium into carbon and oxygen. Carbon is the fundamental structural atom for all organic molecules, including DNA and proteins.
Elements of high atomic mass, including calcium for bone structure and iron for hemoglobin, required more extreme conditions. These atoms were created inside massive stars and scattered across the galaxy during supernova explosions. Supernovae distribute the elements they forged into interstellar gas clouds, which eventually collected to form our solar system and Earth.
Earth’s Material Cycles
Once these ancient elements were scattered into the interstellar medium, they coalesced to form the Earth roughly 4.5 billion years ago. The elements were initially locked within the planet’s mantle and crust, but geological processes, such as volcanism and weathering, slowly made them available on the surface. Hydrogen and oxygen atoms formed water, participating in the hydrologic cycle fundamental to all life.
Biological incorporation involved the biogeochemical cycles, which continuously move matter between the atmosphere, lithosphere, hydrosphere, and the biosphere. Carbon atoms exist in the atmosphere as carbon dioxide before being drawn into plants by photosynthesis. Nitrogen atoms, abundant in the atmosphere, must be fixed by certain bacteria in the soil before being absorbed by plants and integrated into amino acids.
The final step before incorporation into the new life involved the mother’s diet. The parent consumed plants or animals that had incorporated these atoms into their biological structures. Through digestion and metabolism, the parent’s body broke down these complex molecules into basic building blocks, such as glucose, amino acids, and minerals. These materials became the immediate atomic reservoir for the developing fetus.
Biological Assembly During Gestation
The process of building a new human from the mother’s atomic supply occurs over nine months, mediated entirely by the placenta. The placenta acts as the interface, facilitating the transfer of necessary atoms and molecules from the maternal bloodstream to the fetal circulation. It utilizes complex biological machinery to actively regulate the transfer of materials, rather than acting as a simple sieve.
Oxygen and glucose—the primary sources of carbon, hydrogen, and oxygen atoms—are transported to the fetus via facilitated diffusion. This movement occurs down a concentration gradient established between the mother and the fetus. Specific transporter proteins, such as the GLUT family, manage this high-volume transfer of glucose, which serves as the major energy substrate for fetal growth.
Nitrogen atoms, supplied primarily as amino acids, are transferred against a concentration gradient using active transport systems. This pumping ensures the fetus receives a higher concentration of amino acids than the mother’s blood, supporting the rapid synthesis of fetal proteins and DNA. Calcium and iron, necessary for bone mineralization and red blood cell production, also cross the placenta via specialized carrier proteins.
Once these elemental building blocks cross the barrier, the fetal cells rapidly synthesize them into complex biological molecules. The atoms are assembled into strands of DNA, folded into thousands of different proteins, and organized with calcium and phosphorus to create the baby’s skeleton. This assembly converts maternal atoms into the distinct biological entity of the newborn baby.
The Continuous Cycle of Matter
The moment of birth is not the conclusion of the atomic journey but a transition in the continuous cycle of matter. From its first breath, the baby begins exchanging atoms directly with the environment. Oxygen atoms are inhaled and carbon atoms are exhaled as carbon dioxide, constantly cycling the body’s composition.
The elements that built the newborn will continue to be exchanged and replaced throughout life through eating, drinking, and metabolism. When biological processes cease, the human body undergoes decomposition. Microorganisms break down the complex organic molecules—proteins, fats, and nucleic acids—into simpler compounds.
The atoms will then be released back into the soil, air, and water, rejoining the Earth’s biogeochemical cycles. They become available to be incorporated into new life forms, continuing their ancient journey through the cosmos and the biosphere.