杏吧原创

Fossils of single cells have been found in 2-billion-year-old rocks in China. The microfossils may be the oldest examples of complex eukaryotic cells in the fossil record 鈥� in which case they may be our distant ancestors.

Leiming Yin at the Nanjing Institute of Geology and Palaeontology in China and his colleagues found the fossils in a set of rocks called the Hutuo Group in the Wutai mountains in northern China. Previous studies have shown that the rocks were laid down between 2.15 and 1.95 billion years ago.

The team collected samples of slate from the ancient rocks and used acid to dissolve the excess rock, revealing the microfossils.

In total the researchers found eight kinds of microfossil: four were bacteria, two couldn鈥檛 be identified and two appear to be eukaryotes.

Complex cells

Eukaryotic cells are larger and more internally complex than those of other microorganisms like bacteria. The origin of eukaryotes is a milestone in evolutionary history because, while the first eukaryotes were all single-celled, they ultimately gave rise to all multicellular organisms 鈥� including fungi, plants and animals.

One microfossil the researchers found appears to belong to a known genus of eukaryotes called Dictyosphaera. There were also six specimens of a new genus that the team has dubbed Dongyesphaera. Both are roughly spherical cells with multi-layered outer walls and visible spines 鈥� all of which the team says suggests they are eukaryotes, not bacteria.

Experts contacted by New 杏吧原创 gave the fossils a cautious welcome.

It is 鈥減lausible鈥� that they are eukaryotes, says Ma艂gorzata Moczyd艂owska-Vidal at Uppsala University in Sweden.

鈥淚 could go for them being eukaryotic,鈥� says Anette H枚gstr枚m at the Arctic University of Norway in Troms酶.

However, the identification is solely based on the shapes of the fossils, says Yuangao Qu at the Institute of Deep-sea Science and Engineering at the Chinese Academy of Sciences in Sanya. 鈥淚f more geochemical data could be obtained, it would be more convincing.鈥�

Powered by oxygen?

If confirmed to be eukaryotes, the fossils are arguably the oldest known. Previously the oldest confirmed eukaryotes .

鈥淪o far as I know, these are the oldest,鈥� says Qu.

Some researchers have claimed to have found significantly older eukaryotes: reported fungi, which are eukaryotes, in rocks 2.4 billion years old. However, these older microfossils are rare and poorly preserved, and it isn鈥檛 clear that they are really eukaryotes, says H枚gstr枚m. They could be bacteria that look superficially like fungi, for instance.

If eukaryotes really were present as early as 2 billion years ago, they emerged in the wake of tumultuous changes. The first oxygen built up in the atmosphere 2.4 billion years ago, albeit at low levels, in the Great Oxidation Event. This was followed by a global ice age known as Snowball Earth.

These abrupt environmental variations may have triggered the evolution of eukaryotes, says Qu.

However, the mechanisms of this are unclear, says Moczyd艂owska-Vidal. She says that the Great Oxidation Event 鈥渕ight have triggered the evolution of the first eukaryotes鈥�, but adds that this isn鈥檛 certain. Meanwhile, it is even less clear how the Snowball conditions could have contributed, she says.

The early eukaryotic fossil record is still too sparse to draw any firm conclusions, says H枚gstr枚m. For instance, eukaryotes must have diversified in the period after their origin, but the fossil record doesn鈥檛 yet show this.

鈥淭he only certain thing is that these microbes originated in a marine environment with relatively high oxygen level in the surface layers,鈥� says Moczyd艂owska-Vidal.