Life is incredibly diverse, and so are survival mechanisms. While many complex creatures rely on a pumping organ for internal transport, the answer to whether every living thing has a heart is no. Organisms have evolved diverse strategies to move vital substances internally, reflecting environmental challenges and body plans.
Life with a Pumping Heart
Many animals possess a heart, a muscular pump driving fluids through a circulatory system. Vertebrates, including mammals, birds, fish, amphibians, and reptiles, have closed circulatory systems with blood remaining within vessels. Mammals and birds have four-chambered hearts that completely separate oxygenated and deoxygenated blood, supporting high metabolic rates. Fish, conversely, have a simpler two-chambered heart that pumps deoxygenated blood to the gills for oxygenation before circulating through the body in a single circuit.
Invertebrates exhibit a wider range of heart structures and circulatory systems. Many arthropods, such as insects, and most mollusks utilize an open circulatory system. In these systems, a heart or a similar pulsatile vessel pumps a fluid called hemolymph into a body cavity, the hemocoel, where it directly bathes the organs before returning to the heart.
Insects, for example, have a dorsal vessel that acts as a heart in the abdomen and an aorta in the thorax, pumping hemolymph throughout their bodies. Some mollusks, like snails, have a two-chambered heart, while squids are an exception, possessing a more complex closed circulatory system with three hearts.
Life Without a Pumping Heart
A significant portion of life on Earth thrives without a heart, relying on alternative methods for internal transport. Plants, for instance, lack a circulatory pump and instead use specialized vascular tissues: xylem and phloem. Xylem transports water and dissolved minerals from the roots upwards to the leaves, a process largely driven by transpiration, the evaporation of water from leaves, which creates a pulling force. Phloem moves sugars and other organic compounds produced during photosynthesis from the leaves to other parts of the plant, including roots and growing fruits. This transport in phloem can occur both upwards and downwards, unlike the unidirectional flow in xylem.
Simple animals, like sponges and jellyfish, manage without a heart through diffusion. Sponges, for example, filter food and obtain oxygen from water as it flows through their porous bodies, with nutrients and waste exchanging directly between cells and the surrounding water. Flatworms, being very thin and flat, also rely on diffusion to transport substances across their short distances.
Fungi, such as molds and mushrooms, circulate nutrients within their filamentous structures called hyphae. This internal movement occurs through cytoplasmic streaming, where cytoplasm actively flows, carrying nutrients and organelles. Turgor pressure, created by water entering the cells, also contributes to this transport. The smallest life forms, microorganisms like bacteria and archaea, depend entirely on diffusion for nutrient uptake and waste removal within their single cells. Their small size and high surface area to volume ratio allow for efficient exchange with their environment without the need for a specialized transport system.
The Evolutionary Logic of Circulation
The presence or absence of a heart and complex circulatory system is linked to an organism’s size, complexity, and metabolic needs. For small, simple organisms or single cells, diffusion is an effective means of transport. As organisms increase in size, the distance over which substances need to travel becomes too great for diffusion alone to be efficient. This is due to the surface area to volume ratio: as an object gets larger, its volume increases much faster than its surface area.
Larger, more complex organisms with many layers of cells require specialized systems to deliver oxygen and nutrients and remove waste products from cells far from the body’s surface. Animals with higher metabolic rates, such as active vertebrates, also need rapid and efficient delivery of oxygen and nutrients to fuel their activities. The evolution of hearts and circulatory systems allowed for the development of larger body sizes and more active lifestyles, overcoming the limitations of simple diffusion and enabling the diverse forms of life observed today.