Deep Time

Read rock layers like pages and you meet vanished ammonites, trilobites, and non-avian dinosaurs, plus fossils that combine ancestral and derived traits. Branching fish lineages illuminate the origin of early four-limbed vertebrates; one theropod branch survives as birds. Tiny changes across deep time built life's branching tree.

1. Panel 1

   The biologist and child stand at the foot of a vast layered canyon wall at dawn, its countless rock bands stretching up out of sight.

   One round of vary-choose-inherit barely shifts a beetle population. So how do you ever get a whale, an eagle, or a human? With the one ingredient our short lives can't feel: time. Not centuries. Billions of years.

   Juno: Billions? I can barely imagine a thousand.

2. Panel 2

   A giant glowing one-year calendar over the canyon marks strong evidence of life by late March, complex animals in November, dinosaurs in late December, and Homo sapiens in the final 35 minutes.

   Compress Earth's 4.54 billion years into one year. Strong evidence of life appears by late March, complex animals in mid-November, non-avian dinosaurs about December 13–26, and Homo sapiens in the final 35 minutes.

   Wren: Nearly the whole year passes before the worlds we know best appear.

3. Panel 3

   An undisturbed sedimentary canyon wall like stacked stone pages, with a Cambrian trilobite deepest, a Jurassic ammonite above, and a Late Cretaceous dinosaur fossil higher up.

   In an undisturbed sedimentary sequence, lower layers were usually deposited first and are older than those above. Fossils preserve remains, impressions, or traces of past life.

   Juno: So reading downward usually takes us farther back in time?

4. Panel 4

   Three fossils laid on cloth — a spiral ammonite, a segmented trilobite, and a small non-avian theropod skull — beside a modern bird feather representing the surviving avian dinosaur lineage.

   Many fossils preserve creatures now gone: ammonites, trilobites, and non-avian dinosaurs. Birds are the surviving dinosaur lineage. Rock records branches that ended and branches that endured.

   Wren: The world was once full of things now gone. Life keeps changing.

5. Panel 5

   A Tiktaalik-like fish remains mostly submerged in ancient shallows, using sturdy lobe fins to support itself on the bottom; it has scales, gills, a tail, and no toes or legs.

   This fish kept scales, gills, a tail, and fins with sturdy internal bones. Tiktaalik was neither a walking land animal nor a known direct ancestor; its anatomy illuminates changes near the origin of limbed vertebrates.

   Juno: Those are still fins — but there are sturdy bones inside them!

6. Panel 6

   A three-tip theropod family tree places a feathered dinosaur, an Archaeopteryx-like relative, and a modern songbird on distinct terminal branches.

   Feathered theropod lineages diversified across generations. Relatives such as Archaeopteryx preserve one combination of traits, while a surviving avialan branch reaches the birds outside your window. That sparrow is a living dinosaur.

   Wren: Every sparrow is a living dinosaur. The avian branch survived.

7. Panel 7

   A luminous vertebrate tree separates living present-day tips from extinct side branches, with birds nested among dinosaurs and humans on one small mammal twig.

   Across deep time, inherited variation, selection, and chance changed populations. Diverging lineages accumulated different traits, producing beetles, whales, eagles, and us: a vast tree, not a ladder.

   Juno: We're one branch, not the top?

   Wren: One among millions, each shaped over time.

8. Panel 8

   A child traces living cellular lineages backward from right to left toward a braided ancestral population, while earlier history disappears unresolved into mist farther left.

   Trace every living cellular lineage backward and the branches converge toward a shared ancestral population. It was not the first life. Earlier history remains uncertain, so we will follow the evidence as far as it reaches.

   Juno: What lies before that shared ancestral population?