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It is hard to fathom that animals as complex as humans evolved from tiny microscopic organisms. A recent fossil discovery may provide more insights into how exactly the evolution from single-celled forms of life into the intricate life forms alive today happened.
A team of researchers from the Chinese Academy of Sciences and Virginia Tech has discovered some of the world’s oldest multicellular organisms in 600 million-year-old Ediacaran fossils.
The Ediacaran Period occurred between 635 million and 542 million years ago, from the end of the global Marinoan Glaciation to the beginning of the Cambrian Explosion. The study, which was published online in Nature on Sept. 24, offers scientists fresh perspectives on the early evolution of complex multicellular organisms.
In order to determine when, why and how multicellular organisms arose from single-celled predecessors, the researchers examined phosphorite rocks, rocks made up of calcium phosphate, from the Duoshantuo Formation in South China’s Guizhou province. According to Shuhai Xiao, one of the study leaders, the region around the Duoshantou Formation was probably a warm shallow sea 600 million years ago.
The researchers found that the well-preserved, three-dimensional multicellular fossils they retrieved exhibited common qualities shared by complex multicellular organisms, such as animals and plants. Such qualities include cell differentiation and specialization, cell-cell adhesion, communication between cells, the separation of reproductive cells from non-reproductive cells and apoptosis, or programmed cell death.
The organisms preserved in the fossils may possibly be the world’s first animals. By examining the small, spheroidal fossilized organisms, the researchers determined that they could be ancient precursors to either animals or multicellular algae.
The fossils date to nearly 60 million years before the Cambrian Explosion, which was an enormous growth spurt of new life on earth that gave rise to skeletal animals, among others.
The discovery of the fossils provides evidence that multicellularity came about nearly 60 million years before most major animal phyla emerged in the fossil record.
The researchers’ discovery challenges several long-standing interpretations of multicellular fossils. The degree of complexity in these multicellular fossils is inconsistent with the relative simplicity of the bacteria and other single-celled organisms typically believed to be present 600 million years ago. The situation is akin to androids roaming the earth in the midst of mortal humans today.
In previous studies, similar fossils have been interpreted as bacteria, fungi, single-celled eukaryotes, green algae or various types of early animal life. The final category includes transitional forms of modern animals, such as sponges or anemones. However, the scientists believe that the fossils analyzed in this study are most likely not bacteria, as they share characteristics with more complex multicellular organisms. Consequently, this narrows down the possibilities to either transitional forms of modern animals or an ancient prototype of multicellular algae.
Some loose ends remain untied. Since the fossils retrieved from the Ediacaran Duoshantou Formation look so different from anything alive today, they have been difficult to characterize. It is possible that the fossils analyzed in this study may consist of a group of early animals that went extinct and thus have no evolutionary link with the living animals of today.
The researchers believe further investigation is required to pinpoint the exact location of these fossils on the evolutionary tree of life. Xiao suggests that future research should aim to reconstruct the complete life cycle of fossils through an extensive, all-encompassing paleontological investigation.