The long-ago seas of the Carboniferous and Devonian swarmed with tiny plankton that wrapped themselves inside tiny protective shells.

泥盆纪和石炭纪的古代海洋里生活着大批微小的浮游生物。它们躲在小小的保护性壳里。

The clue to this ancestor, it was assumed, would be found in the Devonian era, from about 400 million years ago.

据认为，这个祖先的线索要在大约4亿年以前的泥盆纪寻找。

Well, during the Devonian, things were … complicated.

好吧，在泥盆纪期间，事情… … 很复杂。

Going back even further, we find the Late Devonian extinction about 360 million years ago.

再往前追溯， 我们发现大约 3. 6 亿年前的晚泥盆世大灭绝。

But these creatures lived in a very different world from the Devonian.

但这些生物生活在 与泥盆纪截然不同的世界中。

The Ordovician (440 million years ago) and Devonian (365 million) each wiped out about 80 to 85 percent of species.

奥陶纪（44000万年以前）和泥盆纪（36500万年以前）分别消灭了大约80%——85%的物种。

While fish with jaws first appeared in the Silurian, they proliferated in the Devonian.

虽然有颚的鱼最早出现在 志留纪，但它们在泥盆纪大量繁殖。

But about 385 million years ago, in the Devonian Period, tetrapods started venturing away from the water's edge.

但大约 3.85 亿年前， 在泥盆纪时期，四足动物开始冒险离开水边。

But in the Devonian, a new branch of the cephalopod family tree appeared: ammonites.

但在泥盆纪，出现了头足类 家族树的一个新分支：菊石。

And the evolution of armor may seem like an obvious adaptation for protection, especially in the Devonian's crowded seas.

盔甲的进化似乎是对保护的明显适应，尤其是在泥盆纪拥挤的海域。