From Nautilus, by Amy Maxmen: "The idea of directionality in nature, a gradient from simple to complex, began with the Greeks, who called nature physis, meaning growth." But finding such a gradient is not that easy:
Then molecular analyses did something else. They rearranged the order of branches on evolutionary trees. Biologists pushed aside trees based on how similar organisms looked to one another, and made new ones based on similarities in DNA and protein sequences. The results suggested that complex body parts evolved multiple times and had also been lost. One study found that winged stick insects evolved from wingless stick insects who had winged ancestors. Another analysis suggested that extremely simple animals called acoel worms—a quarter inch long and with just one hole for eating and excreting—evolved from an ancestor with a separate mouth and anus. Biologists’ arrow of time swung forward and backward and forward again.