Scientists in Peru have discovered the skull(颅骨) of an ancestor of modern-day whales. The fossilized skull is believed     1    (be) about 36 million years old. Researchers say the discovery suggests that the animal lived in an ocean once     2    (cover) Peru’s southern Ocucaje desert.

The scientists reported the skull was in good condition when     3    (dig) up last year. It has a series of long, pointy     4    (tooth), said Rodolfo Salas, chief of paleontology at Peru’s National University of San Marcos.

Scientists say the ancient mammal was a basilosaurus, which is in the cetacean(鲸目) family. The name basilosaurus means “king lizard(蜥蜴)” and the animal     5    (think) to have been about 12 meters long. But it was not a reptile(爬行动物) although     6    (it) long body might have moved like a large snake.

Salas called the animal “a marine monster” that may belong     7     a new species of basilosaurus. “When it was searching for its food, it     8    (sure) did a lot of damage,” he added.

Salas explained that when the ancient basilosaurus died out, its skull likely sank to the bottom of the ocean,     9     it was quickly buried and remained.

“Back during this age,     10     conditions for fossilization were very good in Ocucaje,” Salas said.

Ancient footprints discovered in New Mexico indicate that early humans arrived in North America around 23, 000 years ago, researchers reported Thursday.

The first footprints     1     (find) in a dry lake bed in White Sands National Park in 2009. Scientists at the US Geological Survey recently analyzed seeds stuck in the footprints     2     (determine) their approximate age.     3     evidence suggested they were from 21, 130 to 22, 800 years ago.

The findings may help scientists solve a mystery that has long attracted scientists: When did people first arrive in the Americas, after     4     (leave) Africa and Asia?

Most scientists     5     (strong) believe that they came by way of a land bridge, now under water, that connected Asia to Alaska. According to various discoveries including stone tools and bones other researchers have offered a range of possible     6     (date) for human arrival in the Americas, from 13, 000 to 26, 000 years ago or more.

“    7     (we) study provides a more solid baseline for when humans were in North America, although they could have arrived even earlier,” they wrote in the journal Science, which published the study Thursday. “Fossil footprints are more     8     (rely) than cultural evidence.     9     we present here is evidence of a firm time and location.”

Based     10     the size of the footprints, researchers believe that at least some were made by children and teenagers who lived during the last ice age.

3 . Many people today think of the pyramids of Giza as the symbol of ancient Egyptian culture. But who actually built them? For years, we did not know for sure. Now, however, archaeologists have discovered an ancient city near the pyramids. Close by, there is a cemetery (墓地) where the pyramid builders were buried. From studying these places, archaeologists are sure that the pyramids were not built by slaves or foreigners.    1    

It took about 80 years to build the pyramids. Archaeologists believe that about 20,000-30,000 people took part in the construction.     2     Some dug up the rock, some moved it, and some shaped it into blocks. People also worked on different teams, each with its own name. Workers often competed to see whose team could do a job faster.

“We can see that in their skeletons(骨骼),” says Azza Mohamed Sarry El-Din, a scientist studying bones found in the cemetery. According to her research, the bones show signs of arthritis (关节炎)．    3     Archaeologists have also found many female skeletons in the ancient city and cemetery. The damage to their bones is similar to that of the men.    4     Male workers generally lived to age 40-45, but women to only 30-35. However, workers usually had enough food, and they also received medical care if they got sick or hurt.

    5     On a wall in Khufu's Great Pyramid, for example, a group of workers wrote Friends of Khufu. “It's because they were not just building the tomb of their king,” says Egyptian archaeologist Zahi Hawass. “They were building Egypt. It was a national project, and everyone was a part.”