Ever since australopithecus, the ancestor of mankind, began to walk upright 5 million years ago, and the range of activity moved from the forest to the grassland, how to most effectively maintain survival has become a major problem faced by ancient human ancestors at that time. In addition to slowly learning to make tools, preserve fire, sew "clothes", and make traps, finding a place to live in the wind and rain has become a top priority that people must solve at that time, which is similar to how similar we are today. At that time, people were lowly productive, intellectually incompetent, and not yet capable of building durable houses, so choosing the current place of residence was the best choice at that time, and the history of human caves was born and continued for millions of years.
<h1>Strange ancient caves in Brazil</h1>
In the course of millions of years of history, the ancients built and "decorated" a large number of caves, due to changes in geological structure, many caves and "cave" culture were destroyed and buried in the depths of the formation forever. However, there are also a small number of caves inhabited by the ancients that have been fortunately preserved, the more famous ones are: Altamira Caves, Bimobecat Grottoes, China's Hilltop Cave Man Ruins and so on. In these cave sites, not only the "original scenery" of the cave inhabited by ancient humans at that time is preserved relatively completely, but also some of the murals in the cave, the fossils of the ancients and the tools used at that time are preserved, which are the precious "heritage" left to us.
10 years ago, scientists in the southern and southeastern Amazon River basins of Brazil, found more than 1,000 strange caves or tunnels, they are different in width and depth, some of the width of the cave is only 1.5 meters, some are as many as 4 meters, its length is basically about 100 meters, the longest one can be up to 600 meters, the length and width of the cave from the opening to the terminal does not change too much, the rock and soil around the cave is generally smooth. But what is puzzling is that there are very messy scratches in many places.
These scratches are not regular, not like murals carved to record something. The depth of the scratches varies, with the deepest being almost 10 centimeters, as if they were carved with a sharp tool. At first, scientists thought that these scratches were the remnants left by ancient people when they excavated caves, but as the investigation and research gradually deepened, this speculation became more and more untenable.
<h1>These caves are not naturally formed and are not excavated by hand</h1>
On the earth, natural caves formed by natural action can be divided into carbonate caves, gypsum caves and conglomerate caves formed by karst action according to the genesis; basalt caves formed by different solid cooling rates after magma eruptions; sandstone caves formed by differences in rock resistance to weathering; granite caves formed by geological tectonic movements or relative displacement of microplates.
Naturally formed caves, due to the imbalance of the external force, the structure inside the cave will be very complex, the width of the cave or passage will show a completely different state with the change of depth, and the rocks around the cave or passage will also show a very obvious anisotropy under the action of different erosion and other external forces, so that the surface is strange, mottled, and uneven. The internal structure and state of the many caves found in Brazil are too different from the natural caves, and it is impossible to form these very regular and neat caves under natural conditions.
Using carbon-14 isotope dating, scientists have determined that the caves formed at a time that was basically 13,000 years ago. The reason why these caves can be preserved for such a long time is related to the relatively dense and stable rock structure here, and it is also related to the lack of strong geological movement here. Moreover, according to the long-term research of paleoanthropologists, especially through the study of ancient DNA sequencing, it has been found that the history of human activity on the American continent is only about 50,000 years, of which the time in South America is even shorter. The ancients of the American continent, during the Quaternary Ice Age, about 40,000 to 50,000 years ago, reached the northwest of the Americas from eastern Asia through the frozen Bering Strait, and then gradually moved south over tens of thousands of years, until they reached the South American continent about 13,000 years ago.
So 13,000 years ago, south American ancestors just arrived here, or not at all in the Amazon. In addition, the caves discovered by scientists, whether in terms of width or length, are too large, and none of the cave sites left by other ancient humans resemble those in Brazil. Judging from the productivity level of the ancients at that time, they did not have the ability to support such a large amount of engineering, that is to say, the ancients did not need to excavate such a large and long cave, and second, they could not build such a cave at all.
As a result, scientists ultimately determined that the caves were neither naturally formed nor human-made. So, how exactly did these caves come into being?
<h1>Scientists have given new guesses</h1>
Based on the numerous scratches left around these caves, combined with the width and height of the caves, scientists speculate that this should be the legacy of an ancient behemoth, and this giant beast also has strong limbs and sharp claws, and finally scientists set their sights on the ancient giant beast that once lived on the earth 10,000 years ago - the earth sloth.
The earth sloth once lived in Central and South America, existed about millions to 10,000 years ago, and is by far the largest mammal ever seen in Latin America. The earth sloth belongs to the mammalian order Phi Mao suborder Phi Mao, which can reach about 5 meters tall and weigh up to 5 tons, which is comparable to that of Asian elephants. Its limbs are very strong, and the front end of the limbs has 3 very sharp claws, just like 3 huge sharp knives, which can firmly grasp objects and food, but also can be used to efficiently carry out digging operations, coupled with their huge size, strong strength, after a certain period of time to dig out long tunnels and wide caves, there should be no difficulty.
As for why the earth is lazy to dig these caves? Scientists have also speculated that about 13,000 years ago, when the entire earth was still at the end of the Quaternary Ice Age, and although the Amazon basin is now tropical, the temperature at that time was far less high than it is now, especially at night. Even if the earth sloth has no natural enemies, in order to avoid and deal with the threat of cold weather and cooling, it has the behavior of digging holes. The width and height of the cave, combined with scratches on the "walls" around the cave walls, are consistent with the physical characteristics of the earth sloth, further confirming the scientists' suspicions.
With the passage of time, about 10,000 years ago, the end of the Earth's Ice Age, the temperature gradually warmed up, it is reasonable to say that the food source of the earth sloth should become more abundant, but what scientists can not understand is that just after such a turning point, the ground sloth has inexplicably disappeared, as to why it went extinct, there is no reasonable explanation, and scientists need to study it in depth.