Sponges may belong to the earliest group of sexually reproducing organisms, but the first animals to truly mate were not them.
The Earth is home to many animals that reproduce through mating, such as cats, dogs, birds, and bees. So, what were the first animals to do this?
Ocean sponges are among the earliest sexually reproducing animals. (Photo: Federico Cabello).
Sexually reproducing animals have evolved since early times, so the pioneers were also the first organisms to appear on Earth. Researchers are still seeking direct evidence of the first animals, but it is likely they emerged around 800 million years ago, living in the ocean and resembling sponges.
Today’s sponges reproduce by releasing eggs and sperm into the water. These cells then combine to form new larvae, according to the Exploring Our Fluid Earth website from the University of Hawaii.
While ancient sponges may have been among the first animals to reproduce sexually, this behavior existed before they appeared. “The first animals to reproduce sexually were already performing this behavior before they became animals,” shared John Logsdon, an associate professor of biology at the University of Iowa.
To trace the origins of sexual reproduction, Logsdon looked for the presence of meiosis, a form of cell division that produces reproductive cells in eukaryotes—organisms with a nucleus in their cells, such as animals, plants, and fungi. “Clearly, every eukaryote has or had the ability to perform meiosis. The reasonable inference here is that a common ancestor of all of us did this,” Logsdon stated.
So when did the first eukaryotes appear? According to Logsdon, the answer is about 2 billion years ago, when simple bacteria engaged in a form of gene exchange.
Illustration of Devonian placoderm fish. (Photo: Nobumichi Tamura/Stocktrek Images).
However, sexual reproduction in sponges and bacteria is far from the mating behavior that humans and many animals perform based on internal fertilization. To seek the oldest evidence of this behavior, scientists are studying the fossils of ancient fish.
“The oldest evidence of reproduction through mating comes from the placoderm fish of the Devonian period (419.2 – 358.9 million years ago), such as Microbrachius dicki,” stated John Long, a paleontology professor at Flinders University, Australia. Fossils show that male M. dicki had a pair of claspers to internally fertilize females, while females had corresponding genital structures. Long and his research team also discovered that male and female fish would swim alongside each other during mating, with their limb-like appendages connecting.
Sexual reproduction offers many benefits. First, offspring receive genes from both parents, unlike asexual reproduction, where offspring inherit genes from only one parent. This genetic mixing helps animals adapt better to environmental changes.
“Sexual reproduction means that the genetic makeup of offspring is more diverse than that of asexual organisms, which simply clone themselves (e.g., jellyfish). Therefore, it is less likely that an entire population will be wiped out by disease. Greater genetic diversity not only enhances resistance to pathogens but also aids in adapting to environmental changes, including climate change, and can even increase tolerance to toxins if volcanic eruptions change the chemical properties of seawater,” Long explained.
