Analysis puts face on 'cute' distant cousin

It's a small primate with a body shorter than your finger. It has a round face, slender limbs and a long tail.

Cute, right?

Yet the Archicebus achilles is not a pet, it's the relative of a remote human ancestor.

Chinese paleontologist Ni Xijun and his colleagues publish a study on the 55-million-year-old primate fossil in the prestigious journal Nature on Thursday.

The discovery provides insight into the earliest phases of primate evolution, according to a news release from Nature.

The ancient primate's name, Archicebus achilles, makes a reference to the animal's anthropoidlike heel bone.

"Previous fossil evidence shows anthropoids diverged from other primates around 45 million years ago, but our analysis of the new fossil brings the time forward by 10 million years," Ni of the Institute of Vertebrate Paleontology and Paleoanthropology said at a news conference. "It means the evolutionary history of human lineage could be much longer than we thought."

The Archicebus achilles fossil, a nearly complete skeleton, was discovered near Jingzhou in Hubei province about 10 years ago. Since then the fossil has been studied by Ni's team, which used morphological comparison and phylogenetic analysis to search ancient relations to apes, humans and other primates.

"Morphological features are not like DNA evidence, which can be sequenced and compared in a short time," Ni explained. "It takes a great effort for scientists to study the most detailed morphological feature of a skeleton and carefully compare it with a variety of animals."

To reveal more secrets hidden in the rock for millions of years, his team used sophisticated synchrotron CT scanning at the European Synchrotron Radiation Facility to digitally separate the fossil from the rock burying it.

After 10 years of detailed research, Ni and his colleagues studied 1,186 morphological features and compared 157 extinct and living mammals.

"Archicebus achilles is neither a monkey nor an ape. It's such an odd creature that it is virtually a mixture of monkeys, apes and tarsiers," he said.

Monkeys, apes and humans are collectively known as anthropoids in science, while tarsiers belong to a group called tarsiiforms. The two groups have separate evolutionary lineages.

Archicebus achilles has small eyes, short heels and a foot in the general form of monkeys, but it also has many limbs and tooth features typical for tarsierlike primates.

"When the anthropoid lineage began to separate from other primates 55 million years ago, they were actually not so different from Archicebus achilles," Ni said. "But the difference becomes larger as the anthropoids go along one branch, while the descendants of Archicebus achilles go along another.

"The discovery of Archicebus achilles proves that the divergence began at least 55 million years ago."

Deng Tao, a paleontologist of the Institute of Vertebrate Paleontology and Paleoanthropology, said the discovery is significant for humankind to understand more about its origins.

"We can trace back to many mammals in search of our ancestry, but only primates have the closest relation to humans," he said. "The research by Ni and his colleagues found the earliest primate that is very close to human lineage."

Ni also made some innovations in research methods.

"In the past we usually observed the morphological feature of a fossil with the naked eye," Deng said, "but the team applied the synchrotron CT scanning technique, which improved the accuracy to 30 micrometers and successfully uncovered many possible details that could have been missed with the eye."

The team also combined morphological study on fossils using molecular biology, which links the relation between extinct and living mammals.

"We're not able to know the DNA of ancient animals because they are fossilized, but the DNA of living animals, which controls the specific morphological features, are known," Deng said. "The database by Ni's team is unique as it contains both the morphological features and the DNA information of living animals, which helps us find out the relation between humans and other mammals."

For example, tree shrews, a small group of squirrellike mammals, are remote relatives — the evolution tree shows primates and tree shrews separated from each other 60 to 65 million years ago.

Deng said the fossil Ni's team studied was precious, too. "The Archicebus achilles fossil is obviously a treasure," he added.