That’s what acts as a threat to humanity. Her real husband died inside the Shimmer, so who or what came back out?
She later takes blood samples from it only to find that whatever is in the Shimmer has started to affect her DNA. [first lines] 

As Josie Radek (Tessa Thompson) puts it, the Shimmer acts as a prism that refracts EVERY thing inside it. I thought the soldier in the video with the swirling intestines had the infinity tattoo. The ouroboros sign is the representation of regeneration, of the endless cycle of creation and destruction (subtext, annihilation).

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Lena is in a canoe on a swamp located quite near to a corpse that bears the same tattoo as Lena] Lena : Oww. Be it the set of different flowers coming out of the same stem, an alligator with the jaw set of a shark, a weird couple of deer with tree stems as their antlers, or the bear that cries like a human it just ate! When I first saw the trailer, its stunning visuals, a deep reverberating tone and the all-female cast with Natalie Portman at the helm made me excited about it. Did Lena Really Escape The Shimmer? The main one is Lena herself, which we can see as she's being interrogated by Lomax (Benedict Wong). Then there is all the stuff we see in the video-camera. So, the tattoo is just another effect of its refractive properties.The choice of an ouroboros is an interesting one and is actually in sync with the various themes of the film. He and Lena are sitting in their house and talking about his sudden return.

This is also just before she dies.

Also, he must have encountered his doppelganger and that’d have blown the lid on him. They both died at the hands(!) ‘Annihilation’ is one such film. Lomax (Benedict Wong), the guy in the hazmat suit, asks Lena ‘what does it want?’ To which Lena replies that she doesn’t even know if it can want anything.

[the team kill an alligator & examine it's mouth]  And, because we don’t see the disorientations anywhere else in the film, it can be expected that maybe they have grown accustomed to the ways of the Shimmer, now.When Lena is being debriefed, we see a tattoo on her hand.

We see that the ecology inside the Shimmer has gone haywire, with all sorts of inventive blending of the plant, animal and human DNA. [the paramedic, Anya, holds the alligator's mouth open. All images property of their respective owners. So, it is a possibility that in that blackout, Lena and her doppelganger were swapped! So, I was pretty taken by the idea of watching another trick coming out of his box. )It's possible that the tattoo was hidden in the first photo, or that its absence was a trick of the light or a production mistake, but those possibilities wouldn't explain how both her husband and Anya would also have the same tattoo.
It goes from depicting highly gross scenes to equally magnificent views. [as she examines multi-coloured flowers]  Every small thing matters. And we can’t stop that.The beauty of the film is that it is layered with every theme possible, and it makes sense if you choose to see it with a single layer, or with all of them. Like the extinction of one (or more) species and the emergence of the new ones in its place. Lena had no tattoo when entering the shimmer then had a circular bruise in the tattoo area after the fight with the crocodile. She tells Lena that it wants to change everything from the molecular level. In fact, even the lighthouse should be a duplicate. So perhaps, this meteor strike is another overdue change in the ecology and environment. So, before you start debating on ‘the book is better than the movie’ issue, you should know that they are both very different from each other.

With everything inside The Shimmer connected, this kills all the linked organisms.

[She looks at the tattoo on her arm & lifts her arm up]  She walks into the trees, becoming covered in more branches and leaves before vanishing. In other scenes it's clearly absent (such as when Lena takes a sample of her own blood in the village with the trees). An ouroboros (snake eating itself), also fits the theme of self-destruction for creation's sake (as opposed to suicide for destruction's sake). It also represents self-destruction (with the snake eating its own tail), another major theme in the film.We see that after the alligator incident, there is a bruise on Lena’s arm. Audiences may be confused by the sudden reveal, which we talk about below, but you can see the signs from The story of what happens in the Shimmer — an ever-expanding patch of land similar to the Bermuda Triangle where people go in and never come out — is told to the audience by protagonist Lena (Natalie Portman), who has just come back from an expedition into the unknown area.

(Turn up the brightness to see it clearly.

When the group stumbles upon a video camera with footage of what happened with Kane's expedition, they see the disturbing flaying of another soldier. And you’ll know that there is more to it than all that appears.It starts with a meteor hitting a lighthouse and creating a field around itself, its boundary marked with The Shimmer- an incorporeal, shimmering rainbow-ish layer. And at this point, any and all explanations can be deemed right.