A group of researchers led by Shixing Zhu claims to have found complex life in 1.5 billion-year-old rocks in north China. But their paper, published in Nature Communications, instantly provoked intense debate. If true, it would mean that macroscopic complex life could have been on Earth a billion years earlier than previously thought. Until now, the earliest evidence for complex life is the Ediacaran fauna dating to about 600 million years ago. The fossils from this area are mostly soft-bodied animals, although recent evidence points to sponges that may have been a bit older.
Zhu and colleagues found structures up to 30 centimeters (8 inches) long, which they interpret to represent multicellular eukaryotic lifeforms. Based on their morphology, they also speculate that the organisms were photosynthetic, meaning plants or the precursor of plants.
However, the evidence for these “organisms” being either multicellular or eukaryotic is tenuous. Many bacteria can switch between having unicellular and multicellular properties, and some even form macroscopic colonial structures such as stromatolites and microbial mats. A true multicellular organism would consist of at least two different types of cells, and would not be able to revert back to a unicellular state. This is very difficult to prove, especially with fossils. Further, all truly multicellular organisms are eukaryotes, meaning they have a cell nucleus, and organelles such as mitochondria or chloroplasts to obtain energy. They’re usually much more sophisticated than a bacterial cell. All known multicellular eukaryotes are animals, plants, or fungi.
Another study with a similar claim comes to mind. In 2010 El Albani and colleagues published their discovery of 2.1 billion-year-old macrofossils in West Africa. These structures are considered by most scientists to represent colonial organisms. Some chemical compounds indicate that they might have been eukaryotic, but this is very controversial. The presence of animals or plants is not impossible that long ago, because oxygen levels rose significantly after the Great Oxygenation Event about 2.3 billion years ago. But for macrofossils to appear just 200 million years later, evolution would have had to happen at a rather rapid pace, much faster than most scientists think.
The bottom line is that it is very difficult to make a definite call based on morphologies—or shapes—alone. Even if chemical markers are present, as they were in the study by El Albani, it is difficult to be certain. One famous example is the Martian meteorite ALH84001 and the claimed discovery of microfossils, which is still highly controversial and so far has not been substantiated. The case for complex life by Zhu and colleagues in 1.5 billion-year-old rocks from north China is weaker than these claims, because it is only based on morphology and size. So in this case, I side with Carl Sagan: “Extraordinary claims need extraordinary evidence.” And extraordinary evidence is not there for this particular study.