Blood to Molecules: The Fossil Record of Blood and Its Constituents

Contrary to prevalent assumptions, blood—the ultimate “soft tissue”—has a substantial fossil record. Although initial reports of blood remnants from the Holocene were deservedly controversial—and reports of blood cells and proteins in Cretaceous therapods remain controversial today—there is currently good evidence for original blood components in fossils more than 500 million years old. In this review, our knowledge of the fossil record of blood and its cellular and molecular constituents is documented and appraised. Cellular components have been described from both amber (e.g., erythrocytes and protozoan parasites such as Plasmodium and Leishmania) and mineralized bone tissue (erythrocytes and capillary vessels). Although small molecules such as hemoglobin-derived heme and hemocyanin-derived copper are documented in the fossil record, sequenceable polymeric molecules proteins and DNA have the greatest potential for informing us of ancient behavior and physiology—examples include the functionality of mammoth hemoglobin and the disease states of pharaohs.