Tsukiyumi Note #057 Minatogawa Man

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Why did humans start walking on two legs?
How did people 20,000 years ago cross the sea to Okinawa?
What is the reason caves were chosen as the "first homes"?

Mom: "Gangala Valley was great, wasn't it? The feeling of being pulled back into the modern world the moment I walked through the gate that marked the end was a strangely lonely feeling. I want to live there forever. I wonder if I have the blood of Minatogawa Man in me."
Tsuki: "It was definitely good. Maybe it was meant to be preserved because it was discovered in the modern era. I want the test I hid in my room to be found 20,000 years from now and called a miracle."
Mom: "A test? That wouldn't be a miracle, it would be a tragedy. Future historians would mistake it for the intelligence level of the 21st century. Plus, you wrote your name on it, right? All your personal information would be disclosed."
Tsuki: "...For now, I'll believe in a miracle and hide my current answer sheet in a limestone drawer."
Mom: "I'm telling you, it's better not to leave it for future generations! Now hurry up and show it to me!"

"Minatogawa Man" was discovered in an Okinawa quarry in 1970. Because Japanese soil is highly acidic, old bones usually dissolve. However, the "limestone" formed from ancient coral acted as a natural storage container, leaving their skeletons in almost perfect condition. This miraculous discovery reveals the history of humanity's grand journey. Our ancestors began "upright bipedal walking" in Africa about 7 million years ago. By standing on two legs, they could support their heavy heads without relying on neck muscles, creating space for the brain to expand. Then, with their hands freed, they began to make tools, and by moving their fingertips precisely, their brains developed even further. Minatogawa Man was small, with an estimated height of around 150 centimeters, but their lower bodies were very muscular. This is proof that they lived by running around rugged terrain. And they crossed the terrifying sea to reach Okinawa. Even though the sea level was lower during the Ice Age, the fierce current known as the "Kuroshio Current" stood in their way around Okinawa. It is believed that they crossed the sea in dugout canoes by feeling the swells and wind and reading the movement of the stars to brilliantly calculate the "direction they were heading" and the "force of the current pushing them." It was a life-or-death science that deciphered the invisible force of "vectors" through intuition. The safe base they chose after crossing the sea was a "cave." The thick rock walls of a cave have excellent insulation, protecting against summer heat and winter cold. Caves, where the temperature is stable year-round due to geothermal heat, were natural thermoses that protected them from the harsh nature outside, and were their "first homes." The help between companions is visible from the healing marks on their bones. Even without words, they were connected by the power of empathy. The baton of the 7-million-year journey that began when they stood on two legs has certainly been passed down to us today.

Let's try it
● Work 1: Tracing the Footprints of Evolution
Compare the size of your head with that of your ancient ancestors. First, measure the circumference of your head with a tape measure. Next, cut a string to 40 centimeters and make a loop (this is an early hominid). Then, make a loop at 55 centimeters (this is modern man). Standing on two legs allowed the head to be supported straight, which allowed the brain to grow larger. Experience the changes in skeletal structure and brain size.
● Work 2: Designing a Boat That Can Withstand Rough Seas
Think of a boat that can carry a lot of cargo using the mechanism of "buoyancy" that floats on water. Make a boat by folding a 15-centimeter square piece of aluminum foil, float it on water, and place one-yen coins on it one by one. Record how many you could fit before it sank. Try changing the shape by widening the bottom or making the walls higher to see which shape can carry the most cargo.
● Work 3: Survival Through Movement
This is a game where you convey information to your partner without using words. Try to convey the topic "I will catch fish in the river tomorrow morning" to your partner using only gestures and facial expressions without making a sound. Think about how to express the invisible time of "tomorrow" with your body and how to get your partner to have the same image.

The above is an excerpt from the teaching materials I am creating for home study with my son (Tsuki).
[Teaching Material Content] 20260507 Minatogawa Man
・Chemical mechanism of limestone properties and alkalinity in skeletal preservation
・Mechanical changes in the skeleton and expansion of brain capacity brought about by the transition to upright bipedal walking
・Consideration of maritime migration using buoyancy (Archimedes' principle) and the law of vectors
・Insulation effect of cave rock walls and their effectiveness as living bases
・Inference of social structure and communication means based on fossil records
・Grasping direction using the regular movement of stars
・Essay: "Why Humans Kept Walking"
・Debate: "Which is more important for human evolution, feet or hands?"

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