"This DNA sequence has parts identical to humans."
Milia pointed at the tablet.
"Humans? What are you comparing it to?" Kana peered in.
"Yeast. The fungus that makes bread."
"What? Fungus and humans?"
Rei stepped in to explain. "Life evolved from a common ancestor. That's why basic genes are similar."
Kana scrolled through the tablet. "Really... so many matching parts."
"Genes that control the cell cycle. Yeast and humans have surprisingly similar cell division mechanisms."
Milia continued. "That's why we can experiment with yeast. To understand human diseases."
"But why are they similar?" Kana asked the fundamental question.
"Three billion years ago, there was a common ancestor," Rei answered. "From there, the tree of life branched out."
"Three billion years..."
"That memory is carved into DNA," Milia said quietly.
Rei drew a diagram on the whiteboard. "From sequence similarity, we can estimate divergence times."
"How?"
"Molecular clock. Assumes mutations accumulate at a constant rate."
Kana tried to calculate. "More mutations mean older divergence?"
"Basically yes. Though evolutionary rate differs by gene."
Milia showed another example. "Mitochondrial DNA. Inherited only from the mother."
"Not from the father?"
"Not inherited. So we can trace maternal lineage."
Rei supplemented. "Mitochondrial Eve. The common maternal ancestor of modern humans."
"When?" Kana asked with interest.
"About 200,000 years ago. Said to have lived in Africa."
"Someone from so long ago is still in our DNA..."
Milia nodded. "All human mitochondrial DNA traces back to her."
Kana wrote in her notebook. "DNA is a history book."
"Yes. A record of evolution," Rei acknowledged.
"What about older memories?" Kana asked.
Milia operated the tablet. "Ribosomal RNA. One of the most conserved sequences."
"Conserved?"
"Hard to change. Because it's essential for life."
Rei explained. "From bacteria to humans, the basic ribosome structure is the same. Unchanged for four billion years."
"Four billion years... when Earth was born?"
"Almost. Since right after life began."
Kana was moved. "Four billion years of memory lives in my cells."
"Precisely, the function lives," Rei corrected. "The sequence changed bit by bit, but the role stayed the same."
Milia continued. "Hemoglobin is interesting too. Common to vertebrates."
"The blood thing?"
"Protein that carries oxygen. Fish, birds, mammals all have the same basic structure."
Rei drew a graph. "But sequences differ slightly. From those differences, we see evolutionary paths."
"Humans and chimpanzees?" Kana asked.
"98.8 percent match. Diverged six million years ago."
"Just 1.2 percent difference makes us so different?"
Milia smiled. "That 1.2 percent created language and culture."
Kana became serious. "So by comparing DNA, we can find connections with anyone?"
"Theoretically," Rei answered. "In fact, there are projects to reconstruct family trees."
"Amazing..."
Milia closed the tablet. "But remember. DNA is part of the story."
"Part?"
"Environment, culture, choices... they also shape evolution."
Rei nodded. "DNA is the script. But how it's performed changes with time and place."
Kana stared at the double helix model. "Old stories weave new stories."
"Beautiful way to put it," Milia acknowledged.
"DNA isn't just the past," Rei said finally. "The future is written into it too."
Outside the window, the sun sets. Carrying four billion years of memory, life heads toward tomorrow.